I think LTE roaming in Europe is just icing on the cake. I would NOT pick service in the United States because of it, as getting Sprint means you probably won't have LTE service unless you live in a lucky few areas. HSPA+ networks are far more robust in Europe anyway with several networks to choose from, and it's not uncommon to get speeds of 10Mbps down and 2 Mbps up which for traveling should be just fine.
Pick a carrier that works for you in the US, and roam on HSPA+ outside of the United States. I'd save *real* LTE world roaming for the next iPhone when more bands can be supported in the same model (up to 7 from the current 5).
You're right. I am completely basing my choice on the "what if" circumstance that may or may not happen a couple of years from now instead of being concerned about the present. I do have plans to travel in the next year or two but I'll probably just use my current iPhone 4 on 3G abroad after unlocking it of course.
It's just a shame that LTE is all over the place throughout the world and we have different versions of the iPhone (of all phones) to make up for it. I just wish Apple truly made a world phone this time around but I understand that there may have been hardware limitations in the process.