^^ Well mods are endless with both cars, but the GTR being a 2.6lt, inline 6, 24 Valves and Twin Turbo, you will get more top end vs the EVO (stock for stock).
The EVO's advantage would be the weight, as I assume it is lighter.
A GTR will be a decent step up from an EVO too.
With the R34 GTR, if you are modifying it, I would not bother with the V-Spec, just get the base model and work it up from there.
There are slight differences between the V-Spec / V-Spec II / M-Spec but not much until you get to the V-Spec II N1 / V-Spec II Nur / M-Spec Nur that is when there will be a difference.