Actually I don't think the difference is a big deal the only thing is I kinda think the Rim clutch on the 600/620 would be nice. I didn't bother to examine the Sthil 391. My guess is the saw probably has a rim sprocket. I know the cs490 and cs600 does. My Sthil 024 and 026 have them but frankly I grab an Echo nearly every time. The Sthil 029 is heavy very near feels like the cs590 and it's not my favorite saw. MM rim sprocket, professional bar and chain, tweeked and does pretty good but the Echo saws (cs352,490,590) do about all I need. Im semi- mostly, retired just piddling now. Clearing my land and trying to stay out of trees, (72 years old) so Echo saws are fine. I have 5 saws in the shop. Various brands needing attention. A top handle Jonsered and Im determined to die with my steel toe boots on. The Echo's are fine saws and probably the last saws I'll need to buy. Maybe some day I'll take a look at a Sthil 391 or bigger just for kicks. Passed up an old used big (125 ?cc ) maybe, can't rember Sthil. It sold cheap.Besides the bar cover on the echo being plastic (assuming the stihls in mag) what else would the differences be? starter cover, filter cover, top cover, tank and front handle would be plastic on both of the saws.
IF the bar cover being plastic bothers someone they can change it for $35 with a Echo Mag bar cover.