the PK version is hard to explain. It is an inconsistent rip off of XP.
It copies it's uses, but not to the best of standards. If you make games with it, there are certain qualitys of the game you made with it that will make it apparent that you used PK, though it's hard to really spot them unless your game is uncrypted/the person playing is a professional.
I don't suggest PK personally, reasons being it's got a incredible amount of bugs, and RGSS1 barely works, so if you ever dive into using RGSS 1.02 or 1.03, it'd be impossible.
Yes, it is illigal, but on certain sites, it really dosn't bother people >_>
On others, like phanxgames and .org, it is strictly prohibited.
Even if you did buy XP in the end, it would be impossible to actually convert it all. There are some things you can copy and paste, but 30% of the work would need to be remade.
lastly, unless your going to go commercial, I doubt it would really matter. Besides, it's hard to get top notch quality with PK becuase of it's script problems as well as some other function lackingsMy advice is, get XP as soon as possible to explore game making to the best capabilities, or just stick with PK andhave fun.
Then theres VX coming out soon too, lol, could wait for that to come out XD