Echos are good saws.. As are Husky/Jonsered, Stihl and Makita/Dolmar... With this sort of stuff, chose the brand based on local dealer support and the one that'll be in your corner should you need warranty work. After that, it's displacement and power-weight ratio.. A light saw with bad antivibe is at least as bad as a heavier saw.. but it also depends on how much you're going to be using it...
What sort of wood are you cutting? Softwoods I'd look at about a 55cc saw (for 20") while if you get into really tough hardwoods you want to go up to close to 75cc.
Are you looking for a new saw or a good used one?
I don't know the models of the 50cc class saws very well but if you're looking at a bigger one, you really can't go far wrong with something like a Stihl 044/440/441 (different generations of the same saw), or in Orange flavor a Husky 272 or 372... In the blues, the Makita/Dolmar 6100 or it's big bore brother the 7900 is apparently an amazing saw.