My impressions of the two phones:
Droid X
Pros: Cheaper (free), good battery life
Cons: No 4G LTE, end-of-life 3/31, less resale value?
HTC Thunderbolt
Pros: 4G LTE, lock into 4G unlimited plan, better resale?, more responsive?, future-proof?
Cons: costs $200 more, poor battery...