We run and have run quite a few Echo's in our tree business. We have their two largest saws (don't remember the model #'s) and two of their mid-sized saws (4400). We abuse the snot out of the mid-sized saws limbing trees and doing most of the cutting with them, and they still keep running strong. We use the big saws on rare occasion, honestly. We have other saws that cut wood much better. For some reason the Echos don't have as much torque as some of the other saws we run of comparable size. FWIW, we do not use Stihl saws. Their "professional" saws don't seem to last long in our hands. Same for Poulan Pro saws. Echo is just about the only homeowner brand saw that actually stands up to our abuse if that means anything to you.
My favorite saw for cutting wood is the Shindaiwa 757. If you can find one of those for a good price, get it. You will not be sorry. Not only does that saw have gobs of torque, it also zips the chain around really quickly. If I could have only one saw for cutting wood, that would be it. We have bigger Husqvarna saws for wood but the Shindaiwa eats the wood better. I know it's above your price range ($600-$650), but it really kicks butt! The mid-sized saws we use are the Shindaiwa 488 and they also eat through some wood pretty well, but they're mostly our limbing saws.