Great analysis Will.
I finally got to do some work with the 550M2 last Saturday instead of just a few test cuts. 8" to 18" Ash.
The tweener analogy about being in between the Mark 1 and the 562 is accurate. (Return of the 254XP!). By itself, you don't really notice that it's a physically bigger saw. But set it down to the original and you see it right away. ABarrick's pics on page 18 of this thread show it pretty clear. You do also notice the weight. Not much, but you notice it.
Functionally, it's got it all over the original. The heat management, the better air filter, the Gen 3 AT, plus more power, and no flat spot on acceleration. They corrected all the sins from the M1's.........for those that experienced them. (More on that shortly) So, for serious work and something that needs to be relied on all day, it's clearly a better saw.
But for guys who had no issues with the early saws, some may prefer them due to their lighter weight. I had one tree guy in last week who bought one M2 and grabbed himself an M1 before they are gone. (Wants it for climbing.) The original was no slouch in the power department either. Personally I liked the original and it remains a more tossible saw to work with than the M2. (some guys even tossed them out of the bucket when they failed to restart. LOL )
M2 is the better all around saw, as Dustin pointed out. It has a little more reach and can do most of what you need to do. Great for the guy looking for one saw. But how many guys reading this are into the one saw concept?

What I plan to do next is work with one of each, go back and forth and compare them that way. No doubt with all the improvements to the M2 that it's the one I'd spend my money on. But I always liked the M1. Might take 3-4 pulls to restart, but I never experienced a NO start. For me, it's still a nice saw to have around.