The price differense depends on where you are, but the 590 is relatively cheap even here. Despite the 590 has a plastic handlebar and clutch cover, there still is about a pound of weight differense, which of course is important in the woods, but not so much in the yard, if just blocking logs.
I am fully aware that rated max power output isn't the full story, and the 590 will of course cut wood. There is absolutely no reason to believe its "power curve" is any better than the 555 though, rather the other way around.
It just is about how much you are willing to sacrifise, for a lower price. The 620 is better regarding power (3.3 kW/4.5 hp), but also heavier - way too heavy for the power, with metal handlebar and clutch cover. The 555 has both, but still is about a pound lighter than the 590.
The forward part of the tank of the 620 (where the handlebar is attached to it) is known to easily break, and I assume the same applies to the 590.