i bought a 590 a few months back and have cut a lot of wood with it over the last two months.
it is an impressive saw for the money. good power mine has even bone stock. i may test it out with a 24" i think one of my husky bars might fit --
anyway i have been very happy with the 590.
a while back someone had the 620P on sale for $580 shipped i think it was -- had i not just bought the 590 i probably would have got the 620p instead as it is more of a hot-rod and is fully pro class , while the 590 has a few parts that are not quite up to the 620Ps speed. --- i figure the 620P is worth a extra c note or two over the basic 590 if your gonna be working it really-=really hard.
one thing i noticed looking through the parts book is that parts for the 620P are a bit less expensive than a 590 in some cases.
in the long haul a 620P might be less expensive to repair as coils and carbs and so on are cheaper.
i don't think you can go wrong with a 590,600 or 620P as their all first class.