There's alot of factors here.
First off, if you get hurt playing football you signed a contract and will still get paid bank to sit on the bench.
In the UFC, you sign a contract but only get paid if you fight... so once you're injured you gotta get better before you can make more cheese.
Secondly, as others have mentioned, it depends on what position you play. The worst for brain damage is probably a running back, tight end, or receiver. But if you're good at it, you're gonna make sooooo much more money than you would in the ufc.
I would say they are probably pretty close, but would likely think fighters get it worse. Also depends on how smart you are with picking WHO you fight, and how often. The money is so much better in the NFL and you dont have the luxery to chosing to not do your job if you please.
Both suck ass as far as brain trauma is concerned. Thats why you make a shitload of money to do it (hopefully)