Andy.. go the GTR.. you know you want to.
R34s look great on the street, but GTRs have a dominant presence.
In a way, it is a question of show or go, the GTR will always outperform its newer counterpart. Ask yourself, if you are after an almost identical car with some newer bits and pieces, or if you are after a change in driving experience.
Consider too, resale value with the purchase. It is entirely likely that depreciation will occur on a much greater scale with the R34 than the R32. I doubt it would be as extreme as the R33s due to the upcoming change in the import rules, but it would still be on my list when evaluating the two.
You only live once, make the most of the opportunity, go the R