CowayAir Purifiers for Large Room Whole Home Up to 4,253ft², 2 Sets of XL Washable Pre-Filters and True HEPA Filters for Dual Purification, PM1.0 AQI, Turbo Mode, Timer, Wheels, 5yr Warranty, ProX(B)

I bought a 300 for each bedroom on sale and, when they went on sale shortly after, a pair of 400s to cover the rest of the house. Pre-tax ~ 320 and ~ 370 usd, respectively.The 400 is about an inch bigger in each dimension, so size feels the same.The 400 is around 25pct more powerful. 1560 vs 1256 sq ft stated in this listing more or less tells you that, and CADR is there if you dig around each product page on Coway's site (3 CADRs per product).Above were facts you could look up. Now for the stuff I couldn't find before buying.(Note: Coway failed to mention the button differences and denied the sound differences existed when I interrogated them on email repeatedly and then on phone about any/all differences.)The 400 is a lot louder on 2 speed. My phone app standing near the front of the units says 50db and 55db, respectively. With respect to pitch, the 300 sound is a low whoosh and the 400 has a higher pitched undertone. Two household members agree these differences are clearly there but don't mind either sound much; one person can sorta stand being in a room with the 300 on 2 speed but not at all the 400.The 400 is similarly louder than the 300 on 3 speed, though i only measured 2 speed as i wouldnt plan to be in the room with them on high. Both are whisper quiet on 1 speed.The two units have the same power button and same light/timer/etc buttons on the left. But the buttons you really use the most are different. Look closely at the pix and you can see the 300 has one button (between the power button and the s for smart) while the 400 appears to have no buttons there. Actually, the 400 has a series of non-labeled buttons in a little arc - separate buttons for smart, night, 1, 2, 3. I thought, "oh this is much better" when discovering this upon hooking up my first 400. But, definitely there are more failure points and these buttons seem each to be inherently less likely to last forever than the type of button used on the 300 (where i have to press/beep several times instead of just once to get the setting i want). Then I noticed that the 1 speed button on that unit only worked less than 10pct of the time. I returned that unit and got a replacement 400... that was immediate and before I used them at all and when I just knew I wanted the larger model for the non-bedroom rooms; having had both models for a week or two now I'm not sure which i'd go with if I had that decision again.My cats activate the buttons on both units when the stand at the units like a counter, and when they use them as "butt coolers." There is no control lock. I plan to cut up a 1/10" plastic sheet from HD and drill/countersink button access holes that are big enough for fingertips but not big enough for paws. I might also lightly spraypaint the underside of that plastic, as the indicators are very bright and cannot be dimmed.On both, the lights (except the smart/night/1/2/3 lights) turn off at one button press. You can't turn off the beeps.Both need the prefilters cleaned weekly or so - takes seconds to pop the sides off and do it with the hand brush attachment on a vacuum.In my limited experience, the 400 has a better sense of smell - it kicks on more often in smart mode. (I have all units set to maximum sensitivity.)On both, you can use a smart plug if you want. It starts on the same setting as it was when it lost power. On the 400, I probably nevwr will but you could use those button bots if you wanted - eg, one for 3 speed and one for 1 speed. This wouldnt work on the 300 since it's just one button and couldn't keep track of the setring (unless you also had a camera, i guess...).One comment that isnt a comparison of 300 and 400...After my very brief experience with Mila prior to finding Coway, I have zero appetite to try a purifier with built-in connectivity. Mila was much louder than either Coway, buttons werent responsive, manual speed had a known bug that it would change itself, power up took minutes not seconds and didnt resume previous speed, it couldnt smell anything even if i shook out pine pellet cat litter right next to it, and a few other things. The reviews have me convinced that Coway smart is also not fun. I can get all the convenience and control I could possibly claim to need - and a device with less in it to break or annoy - with a $7 smart plug (or without one and let it run on smart or 1 or 2 nonstop).I doubt there I'd a product is prefer. Gimme control lock and dimming on the speed lights and it is perfect.EDIT after a few more weeks: another 400 was also defective. Rhythmic, loud noise on 1 speed that would sometimes find its stride/resonance/whatever and be heard all around the house. Amazon limits qty of 400s per account; due to defects, I had to switch to my spouse's forgotten, linked account in order to buy the 4th one. 300s: 3 for 3... 400s: 2 for 4. Also, I figured out how to reduce the volume overall and practically eliminate the high pitched noise on the 400... either add very thin weather stripping to coway filters (HD has a 17' roll of 1/8" foam that is very compressible, will try applying that on next filter change but should work).... or replace them with cabiclean filters for 400 which are a snugger fit and 1/3 the price but otherwise identical (I did this for now, bought backup cabicleans in case something changes with price/availability and set aside the originals). I started using smart plugs as timers for all units; the blue light on front defaults to on which can be annoying in bedrooms, but easy to dim with some tape and a piece of copy paper (folded in half and then thirds seems perfect) after popping off the front cover. Lastly... the 400's far left light is a white ring that starts to disappear as an indicator that time to vacuum the pre filters is approaching; the 300s turn orange (either orange or not, no gradual indicator) when that circle on 400s is still more than 2/3 full. I do all 5 once one goes; in my house, they all appear to (equally) need it at that point.

This unit seems very accurate. Every time my wife cooks, it kicks into high speed from the smoke. I'm not joking, it actually does. Even if it's just a strong smell from the cooking, it goes into high speed. I even heard it kick in one time after I sneezed. I've already cleaned the outer filter 3 times since purchasing it (about a month now), so it's collecting. And we no longer see the dust floating around when the sunlight shines through the windows in the morning. It's made a big difference.On the cons side, I see no reason for this thing to make a sound when it changes mode. When I configured the auto mode change at night and in the morning, it make a bing-bing sound 3 times in a row about 10 seconds between. It's in our living room, so no big deal, but if you plan to sleep nearby, it would be annoying.Another con, the app is not that great. I had to disable it sending alerts, because the only alert it sends is that your air quality is extremely bad. And it did it so often that I finally disabled it. But then when I opened the app, and viewed the air quality readings, it was never in the extremely bad range, or only hit it once or twice, much less than the numerous notifications.I would recommend going with the non-app version which is significantly cheaper. You won't use the app, unless you forgot to set the mode of the unit before leaving on vacation, but even then, what's the worst that will happen? If no one is in your home, it's unlikely there will be any dust being stirred up anyway, and it will just keep running in the previous mode. The app does show air quality in a graph form, but looking at that wore off after the first couple of days.The build quality is really nice. Heavy strong plastic sidewalls, easy and intuitive filter replacement, and it's very quiet for the amount of air it appears to be moving. Overall, I'm very satisfied with the quality!