I have the choice of either 15T or 15U in the Army. Which will be better in the long run, as in which will be around longer?

If either airframe is retired, you'll be transitioned to a new MOS, most likely for the airframe that replaces it. I would not let this worry you. This happens on the ground side with MOS alignment, consolidations and new equipment programs. It doesn't happen that often.

They’re both good MOS’s I chose 15T because we have more duty station options than 15Us

As far as which airframe will be around longer. I doubt the Army will get rid of either of them. The 47 is going to be a 100 year airframe. All 4 branches of the Military use the 60, and for good reason. It's a great aircraft, very versatile. If you enlisted today at 18 years old, and retired at the maximum age to serve of 62, both airframe will still be in service guaranteed