Honestly, I was shocked to see this thread, because these two places could not be more opposite.
First, I agree with what Dragonfly is saying about clinical research as Mayo and basic science at UTSW.
Mayo is definitely more cush, but, in my opinion, is one of the few programs on the cutting edge of graduate medical education. Meanwhile, UTSW is still in the dark ages in this regard. Now, you just have to decide what you want. I think most of us are seeking a golden mean. But, some people want a more relaxed experience, and some feel they need to get beat down daily to get a sense of acomplishment.
Also, in terms of location, these could not be more different. Mayo in in freezing cold Rochester. A very good place if you are married/with family. Meanwhile, UTSW is in Dallas, a pretty cool city, a drastic weather improvement, and I would imagine a good singles scene.
So, if I'm being honest with you, I find it hard to believe you don't know which side of the fence you fall on here. It's like choosing medicine vs surgery. It should be obvious what general environment you want to train and live in. Start with either GME ideology or location and build your preferences from there.
These places are both well known, and you'll get whatever fellowship you want coming from either one. Both offer very solid IM training but in very different ways. Where do you fit in? Where do you want to live? If you have any ideology of how GME should be carried out, it should align more closely with one of these over the other. Give it some thought.