Go with the 620 over the 590. It has a 2 ring piston, better coil with more timing, and wakes up nicely with a muffler mod. They really wake up with mild porting.
If you’re ok with the extra weight over the 400, and value the simpler design and adjustable carb, the 620 is probably for you. The $ saved over the 400 can be put toward more chains, bars, maybe porting.
If you’re going to leave it stock though, and have the $ for the 400, it’s really the better saw. The power/weight will be noticeable on your back once past a couple hrs cutting continuously. No having to think about carb adjustments over time with the Mtronic either.
Jason