I have a 2 piece by FireFly and LOVE it. It is so easy to take off the top to pack and then if you want to go up in just your pants, you can. It looks just like a one piece when you are wearing it and I have never had any trouble with the top riding up.
I think the concensus from here so far is therefore that you should spend it - ie, everyone's saying having both is better than only a pair of pants or a full suit. The whole point of the 2-in-1 is that it IS both. (its only like £8 at the mo anyway)
See if anyone comes back with a comment on how good a 2-in-1 works as both though, maybe its a case of "jack of all, master of none"?
I had a full suit and now I purchesd a 2 piece so on hot days I can jump with only the pans and a t-shirt, It gets very hot in the summer here in Israel. ppl on my DZ that have 2 piece are very happy about it.