UnderArmour is cool for football, but from a perspective of what a university like Tennessee needs in an apparel contract, they are incredibly weak. You have to factor in outfitting every single athletic team, as well as providing other benefits to the university. (Here in the north, for example, UA would fail miserably with hockey.)
There really are only two players- Adidas and Nike. At Michigan we were with Nike from 1995-2007, and Adidas from 2008-present. Each has their advantages, and I have experience with Nike as they outfitted our team while I was part of the program. Nike gear really was top notch. The only thing I didn't like as much was the fact that they started tinkering with the uniforms in 2005 and created a weird Miami-esque template. When we changed to Adidas, I thought it was kind of cool, except the amount of merchandise available to the fans was decreased. Adidas has less lines. Michigan, Tennessee, Notre Dame...they are all "top level" Adidas schools, but the amount of stuff available pales in comparison to what Nike made for "platinum" Nike schools when we were under contract with Nike.
The thing I always liked about what Tennessee does (that is the result of NOT having a messy split with Nike like Michigan) is stock retail lines of both Adidas and Nike. Tennessee fans can buy all Adidas merchandise and also is I believe still a "gold" level Nike school for merchandising. That is really cool, and I always looked forward to going to Knoxville and being able to get the best Nike and Adidas UT things.
From a player's perspective, Nike performance gear was a little better, but Adidas was much less controlling as a brand and from what I hear delivers exceptional customer service.
One thing I wish both brands would go back to is uniqueness between schools. Kill a lot of the generic templates, or at least use more school-specific things. I liked the late 90s when you still had the quality and consistency that an official supplier provided, but weren't saturated with identical uniforms/sideline apparel/fan gear. Now, when I go to Knoxville, I'm buying the exact same design of shirt from Adidas that I already have in Michigan, it's just orange and white with "TENNESSEE" on it instead of "MICHIGAN."