Well...
Is your area going to get 4G anytime over the next 2 years, and if/when it does, will you want to use it?
Cause right now, you'll be locked in for 2 years, and you'll be paying the exact same price per month for either the TB or DI2. However, with the TB, you'll get 4G once its turned on, and thats something that you will not experience with the DI2.
Price per month, it makes more sense to go with the TB. You get more out of your plan for the same price. Upfront costs might lean toward the DI2, though, but likely only slightly.