I just received my used (1.7 hours, oh no) BiPAP Auto today, and the differences are more striking than I imagined. For each category, I'll cover the M Series first and then the Tank and then I'll crown one the winner of that category. I won't address algorithmic issues because obviously my BiPAP Auto is more comfortable than my APAP.
Size/Weight/Travel
This has been talked about ad nauseam on the forum, so yes, the M Series is smaller, but mainly it's the height factor that makes the tank seem so rawr to me.
Winner: M Series, especially if you don't need a humidifier. It's lighter too.
Humidifier Design
This may sound strange, but to me this is probably the single most important part of my CPAP experience. I use my humidifiers on max, so I nearly empty the tank (well, the M series one anyway) every single night, and filling the chamber is a very regular part of my life.
The M Series humidifier sits off to the left of the actual machine. You can separate the two pieces by grabbing under the front of the actual flow generator and pulling up gently. You'll feel a pop and it'll come loose. In order to use the machine without the humidifier, there's an included plastic piece.
There's a large obvious dial on the top of it, and that's how you adjust the setting. This is a touted feature of the M Series, but I'm not that impressed. I don't exactly sit there and crank through the different humidifier settings each night. In fact it's become an annoyance. I sometimes go to bed, feel my throat incredibly dry after about 10 minutes, and then find the dial on 3 or 4 since I bumped it while I was grabbing my mask.
In order to fill the M Series humidifier, you open the door on the front and pull the tank out from the machine. You then fill the silly thing holding it sideways, and the fill line is just BARELY below the opening. You then tilt back flat and put it in. The humidifier turns on and off with the machine and there is no way to preheat before you turn on the flow generator. One night on the M Series I filled the tank only to hold it a bit wrong as I was tilting back down and dumped water all over the night stand.
The easiest way I found to fill the sucker was to get one of those 2.5 gallon bottles of distilled water that are meant to go in the fridge with a spigot. Awkward to pour out of a gallon jug.
Another note here, the little window in the door on the M Series machine is there to supposedly let you see how much water is in the tank. In order to see this window at all, I have to bend over, and then once I do, there's not much to see anyway. The entire tank on the Tank (sorry for the pun) is transparent, which is more medical looking, but a much more practical design in my opinion.
The Tank's humidifier sits in front of the machine itself. It definitely has higher capacity than the M Series. In order to fill, you remove your hose from the tank and pop in a funnel. It's much easier to fill because there's not the other half of the tank preventing you from getting a good angle on the opening. I'd recommend getting ahold of an elbow so that you put less stress on the end of your tubing.
To turn it on, you press the heat button, and you can preheat before you actually start, which to me is VERY nice. I've generally ended up with a dry throat at the beginning of my nights which has kept me up until the water warms up properly on the M Series since you can't preheat.
To adjust, hold the heat button until the display changes, then choose a number. Harder than the M Series, but how often do you adjust humidifier settings anyway?
Winner: Tank. By a LOT.
Memory Card Slot
The M Series card slot is located vertically on the very back of the machine. Tank has it on the side and horizontally. It's a lot easier to get to on the tank. Granted, the typical user doesn't care, but we certainly do! Reaching behind the machine where the cords are every day or two is annoying.
Winner: Tank.
Noise
This entire section needs to be prefaced with the fact that I'm comparing apples to oranges. The tank I have is a BiPAP Auto while the M Series is an APAP.
That being said, I was FLOORED by the difference in noise. Here are recordings of the APAP operating at 15cm h2o and the BiPAP running at 15/15cm h2o. C-Flex and Bi-Flex are turned off on both machines.
http://cpap.blargity.com/m_vs_tank/MSeries.wav
http://cpap.blargity.com/m_vs_tank/Tank.wav
(I'm leaving them in wav so you can real sound, not a compressed approximation.)
Yes, I'm breathing via the mask in BOTH samples.
Edit: Apparently the Tank APAP still has the varying pitch. Maybe someone will be kind enough to record from their Tank Auto so we can get a general idea. Of course the sound varies from machine to machine even when they're the same model, so yeah. Not scientific, but might help anyway.
For the newbies that have never heard a CPAP machine, I amplified both sounds using my computer so you could better hear the difference. I know they both sound like a vacuum cleaner, but in real life they're nowhere near this loud.
The M Series has this high pitched whine that I hate compared to the Tank. The sound also varies in pitch much more between inhale/exhale. Remember, this is with C-Flex off. It's even worse when C-Flex is on. I wouldn't go so far as to say it's louder, but it's definitely higher pitched, and to me, more piercing.
I used to set my ramp to my min pressure, and if the auto decided to go up to about 17 before I fell asleep, I'd hear a really REALLY annoying whine, so I'd hit the ramp button just to get it to hush.
Winner: Tank. By a lot. (Apples and oranges though. See above.)
Displays/Menus
To me this isn't a huge deal. You'll get a machine and you'll learn it one way or another. It doesn't much matter how intuitive the menus are as long as they work and you can decipher them initially. That's what this forum is for.
The M Series has redesigned menus. The Tank doesn't. Here's the same setting and what it looks like on both machines' screens.
What the heck is start? What are the possible values? You get the idea. This is the single most confusing setting on the Tank though, so don't get too concerned.
The M Series also shows week average AHI and Leak data on the display without the need for any software. To me this is pretty pointless because that data isn't very helpful at ALL without the full detail report. You know your AHI is 12.4. Fantastic, how do you fix it? Do leaks look good? Yup, they do. Who knows what's going on unless you have the software? Any newbies reading this, don't be tempted to go, "Oh gee, I'll just get the M Series and then I won't need the software." Not true at all.
Winner: M Series.
Brightness
It deserves mention that the M Series has incredibly bright blue LEDs under the buttons that stay on while you sleep. They actually light up my ceiling. The Tank (this one anyway) has lights under the buttons but they aren't that bright, and you can actually turn them on or off to your liking.
Winner: Tank. Sure, blue is stylin' but I'm just trying to get some shuteye.
Summary
The M Series is a good machine, and has served me well. I find some issues with it annoying, particularly the humidifier situation and the high pitched noise. It also deserves mention that I'm on my second M Series humidifier due to an issue where they just flake out and stop heating the plate for no particular reason after a couple months.
I can't address reliability with the Tank, but many here can. See their posts.
Recommendation: If I had known the difference in humidifier designs would be this dramatic, I would have complained when I got an M Series. Get a Tank. You can get your travel machine later.
