I tried to create a 16v out of my old 1.2 (1995). Basically, youre better off making something that looks really nice, but doesnt go as well as it should! In the end I bought a 16v, because I was going to have to spend way too much to get what I wanted.
What I know is that the 16v is markedly different from a 1.2/RT, both to look at and under the skin. I would imagine thats the case with the 172. Renault will have moded all the things you can think of - and more - for much less than youll be able to do it for.
If you want an individual looking car, then do up your 1.2. But if you want a 172, then save up! Im not going to get into the whole "is a 172 more fun to drive than a 1.2?" debate as I think unless youve driven both then its not right to comment.