Neither, get a Mk2.
Mk3 is by no means rare, they are diminishing in numbers but this is due to most of them being rotten. If you do need to scratch the MK3 itch then make sure it is a 16v, avoid the 8v.
The Mk4 is a big lardy car, which doesn't really handle/perform well in standard guise, you'll need a remap and some decent suspension to have any fun in it.