Are you the same person who wants a 57-plate ED30 at less than 30k mileage for under £12,000?
If you're gonna smack it around on a track, does it matter about the mileage? Why 57-plate and not say 07-plate (which is when ED30s first came out)?
As for GTIvED30, apart from power, identical in handling so it's more about the power. Also depends on how much you plan to mod it IMO. A standard GTI will be cheaper so more £££ saved for more mods.
As recommended above, try them both at stock and see what the difference is.