Id go 3.9 Stu.
Punch the ratios into a gear ratio chart...there's a few floating around on the net.
Std Gti ratios are 3.45, 2.12, 1.44, 1.13 & 0.91 and 0.91 if its a 16v box...0.89 if its 8v.
Pop in yr tyre size and max rpm and see what you get...look for around 35 in 1st, 60 2nd, 80 3rd, 105 4th 115-120 5th.