With regards to pro football, throughout my studies on TBI, I'm quite confident to say that almost every other sport is "safer", as the concern is the repeated shots to the head as opposed to the one that actually puts your lights out. In that sense, similar to football, pro boxing is widely believed to be more overtly dangerous than MMA, as boxers absorb so much more consistent damage over their careers.
PBS did a pretty engaging analysis in an episode of Frontline for those interested in a cliffnotes take on head trauma in football.
All that said, I defintately wouldn't put MMA on the "safest" sports side, but I'd hazard to say that the culture of football is more dangerous.
MY MMA RANKINGS
HW Velasquez Cigano Werdum Cormier Bigfoot Barnett Mir Browne
LHW Jones Machida Evans Hendo Davis Goose Teixeira Shogun Mousasi Bader
MW ASilva Rockhold Belfort Weidman Jacare Okami Kennedy Consta Belcher Boetsch
WW GSP Hendricks Condit MacDonald Diaz Maia Fitch Kampmann Ellenberger Pyle
LW Bendo Alvarez Maynard Melendez Thomson J.Miller Diaz Pettis Chandler Grant dos Anjos
FW Aldo Mendes Curran Lamas Frankie Zombie
BW Cruz Barao Fernandes Faber Moraes