They both do pretty much the same. X2 doens;t have as much layers options as PS, for example whereas you may apply a sharpen layer in PS and use masks to selctively sharpen, that's notr possible in X2, but you can use the sharpen brush to achieve the same end result, it's just not on a new layer.
I think PS is a more one stop shop, from Raw to output and everything in between. X2 doesn;t support my Rawfiles, so I do most if my work in Silkypix, then if there's something that absolutely has to be done in PSP, it's done afterwards.
I like PSP very much, I think XI was a slightly better version though. Haven't tried X3.