Blue, you are absolutely correct!
And personally, I've been able to do more than 40 dives on a single set of AAA batteries in my M2, so changing was no hassle. Plus I use rechargeable AAAs... So it's cheaper and you don't throw them away after!
To the best of my knowledge (Phil, Zak correct me if I'm wrong), the puck uses the same RGBM algorythm the M2 does. The only real advantages I see in the puck over the M2 are: Easier to push button (the M2 buttons can be hard), easier menu, longer lasting battery (if changing a battery is a hassle for you
), and it's the currently supported Mares computer (M2 discontinued). Other than that, it is pretty much the same as the M2!
On the other hand, user replaceable AAA batteries in my book are a big advantage. Also, the M2 has a permanent backlight you can keep on during a night dive (I love this function, not many computers do it!!!). It's cheap (179$ at Scubatoys) and Mares warranty still applies even if it's discontinued.
If I had to choose, I'd buy another M2. But both are good product and I think it's more the fine details which one person will prefer over another, which are the difference.
Just food for thought!