Whatever you decide it's up to you, no right or wrong, different circumstances etc. Here's my interpretation of personal finance on cars, you should know this before deciding.
Using VW.co.uk website, their R7.5 offer will cost you over three years:
Deposit: £5,158
36 payments of £421: £14,735
Insurance at £500 a yr: £1,500
Total cost: £21,393
Value of asset at end of 36 months: £0 as you give the car back (or pay £14,465 to keep it).
Buying a used mk5 gti, cost over 3 yrs.
Cost £5,000 (can put on a 0% credit card and pay monthly - no need to give bankers even more money)
Service £250 x3: £750
MOT £55 x3: £165
Insurance at £400 x3 £1,200
Two new tyres: £160
Misc, brakes etc: £400
Depreciation: £1,000
Value of asset (if you sold it after 3 yrs): £4,000
Total cost: £3,675
If I'm correct then the difference in cost over three years is £17,718! That could be enough money to take a year off work, or retire a couple of years earlier. Or spend on 3 yrs driving an R
Your choice, all I'm saying is have a look at your finances before you decided.