dunno why ure saying a valver or 106GTI. 106 costs more to buy, so valver is gonna be cheaper in that respect. Not thought about a 205GTI 1993 L reg or something like that? Group 13-14 insurance (last i saw... could be wrong) and very quick cars and handle better than both of those. Depends what you want I guess, driving experience or newness, refinement, reliability etc... Check the running costs of anything you buy, I find my biggest expense is my petrol consumption. 200 miles outta a full tank seems pretty bad... Insurane is another concern at your age, ideally you are better keeping your current car until your 21, then you have even more options. Why compromise when you can be looking at 306 GTIs, Cups etc etc.
Good luck in whatever you choose tho m8, end of the day decide what you use the car for and how much money you wanna spend on it.