The 55 is not anywhere close to a cs590.
Like I said I like the 590 a lot and I have ran a 55 that ran very well and I don't believe there is enough difference to warrant going from a 55 to a 590 but that is my opinion and everyone has one. If the OP is just looking for a new saw then by all means have at it, personally I think if he feels he is needing to step up to something more powerful then I would recommend a 70cc class saw. If however his 55 isn't performing up to what I have seen one do then he will be happy I'm sure with the 590, just remember to dial in the carb. once you get it and there are a ton of threads on that in here.
Anyway to the OP have you decided on what you want to do? heck this forum is no different than any other forum out there. Ask ten people and you will get ten different answers . I kind of find it interesting to see what people end up deciding on.