The Pro line is their professional product. You probably don't need it, unless you do, which if you were a professional you would know. Lol, sorry for the sarcasm, but seriously, unless you're sure, get the Movie Studio line, as its very good for its price. And don't bother with the Production Suite unless you know you need it, its quite a bit more expensive than the regular HD Platinum version and, in my case at least, I would have no use for the added features. And the Sony Vegas web site has a very good comparison chart showing what each version (Pro included) offers compared to the others. Also be sure to utilize the free trials if you're not sure.
This is all general advice, as you haven't been very specific about what you were planning to do with the software.