I've had my share of 50cc saws and currently own 6 at last count. I've ported a few I've owned, and a few for friends, some are older saw some newer. I won't say much about the older saws, those are more for the collectors, and parts aren't always plentiful.
When it comes to the more common models here, yes I'll go with the flow on this one, even though I usually goes against it. My favorite 50cc saw, really comes down to a group of saws, 351,353,346OE and NE.
The Stihl 260 is a good saw in its own right, similar in some ways to the old 50cc Poulan saws of the same era. Simple and tough, but not necessarily a powerhouse.
The 5100 Dolmar is by far my least favorite modern 50cc offering for a number of reasons, I'll leave it at that.
The MS261 is a really well built saw that has a lot going for it, especially as a real firewood saw. An underrated saw here, and for no good reason I can come up with, well maybe it's stupid Stihl price tag. The balance and weight group sure have beaten that horse to death.
The verdict is still out on the 550. Fit and finish is lacking, the air filter on mine doesn't fit properly so it sucks in dust like crazy. The transfer seals will be coming off to prevent an inevitable air leak, mine has the older poorly molded seals.
Have yet to run the Echo, no doubt that at $350 it's a serious contender for bang for your buck.
Anyway that's my take on things, for whatever it's worth.