Cylinders are a little different port timing between the 620 and the 590. I run both and the 590 can be made to cut very close to the 620 with no port work.
The larger carb and the ignition coil of the 620 is unescassary on the 590. For a $400 dollar saw, 90 minutes of work, the 590 out-cuts a $900 stihl ms 362. So at less than half the price, it's the most versatile, best value out there.
Just tooled up a 590:
Redbeard bar adaptor(stihl bar to 590/620). Free Versacut aluminum-core bar I found in a sawdust pile. Exl chain off my 3 yr old spool.
Walbro 86-578-1 fuel nozzle, carb limiters filed off.
Clutch-side felling spike ( oem echo acsessory).
Base gasket delete, using threebond 1207d sealant on the base.
.015 off the flywheel to advance the ignition timing. Don't like anymore ignition advance than that on the gas i get hear in Alaska. Some of these youtube goobers, you can hear "cackling" in the saws in the cut: too much ignition timing.
Muffler opened up.
Anyhow, a 590 from this seller, you won't do any better. Most smiles per dollar.......