I don't think he'll catch Tiger though. There are too many others that will challenge, Johnson, Mcilroy, Rahm, Day, Thomas. Can't see anyone truly dominating, but can certainly see Spieth winning 5 - 8 in the next 10 years.
If McIlroy doesn't suffer Tiger type injuries and stays fit, he'll get a lot more. Nowhere near Nicklaus or Woods but he'll be the most successful British golfer of all time. If he does suffer injuries I'd go for Spieth
Spieth is by far the steadier golfer, and will grind out more wins. Rory, IMO, is another Ballesteros. He will win spectacularly and crash spectacularly too. I'd rather watch Rory on a good day...
When I think about the next 10 years, you have to feel that Speith will always be in with a shout at every major, no matter what the course is, because of his amazing wedge play and putting. I think he is more than capable of going out and doing the grand slam at some point.
I'd suggest that Spieth and Thomas both look more comfortable under pressure on the greens than Rory.
For that reason alone I'm going with team USA. Spieth is my call too. Just looks so much more consistent than Rory. Then again I wouldn't be surprised if it was Thomas.