If you do your research... know what you're getting into and budget for it, both are good cars. Just make sure you get a good check on one.
If you get a lemon, a GTR or GTiR will cost you an arm and a leg to fix. I know someone who rebuilt his GTiR engine. It cost him bucket loads of money and took him months to find the correct parts. Some where hard to come by. A GTR needing a rebuild will be very similar. Parts may be a bit easier, but still cost a lot.
As a few have said, maybe go for something like a gtst. They're a but cheaper and more common. You said you are putting it on your dad's insurance. How old are you? Is this your first car? If it is, both the GTR and GTiR can be a handful for an inexperienced drive.
Good luck with the buy, but just know what you're getting into.