I picked TJ but honestly there are enough questions surrounding both men that it is a tough bout to call with any confidence. I think a lot of people are forgetting that TJ's striking is technically very solid and the same things that Font troubled Aldo with can be utilized to a much greater degree by Dillashaw.
He is quick with odd timing and rhythm and Aldo appeared to have trouble with Font's speed and timing. Aldo's pivots are the basis of his defense and Dillashaw is very adept at utilizing lateral movement and shifts to win the foot battle himself. Font touched Aldo up even after Aldo found his timing and began landing hard shots and TJ doesn't just touch dudes, he's got solid pop and oftentimes hurts them badly. TJ also seems to still have the neverending gas tank and pressure and too much defensive grappling and a high pace have bothered Aldo before. If TJ fights at his customary pace and can feint Aldo with strikes and threaten the takedown to get big reactions early I can see Aldo being overwhelmed. Dillashaw brings tremendous striking and grappling and can legitimately threaten Aldo in both areas, so if he can get Aldo to overreact to one and underreact to the other he can put him on the back foot, force him defensive, and wilt him with the pressure and pace.
The biggest thing in Aldo's favor is his own significant power coupled with Dillashaw's very shaky chin. If Aldo is able to catch him early TJ could find himself in deep shit or staring up at the lights early. Aldo also has the best weapon possible to counter a guy whose striking game is based on footwork and that is namely the baseball bat low kicks. He could find it hard to catch TJ with solid punches early when he's circlung or darting in and out with his shifting punches but the legs will be there to be hit throughout and Aldo doesn't need a whole lot of solid connections to greatly impair TJ's movement. Dillashaw's striking is heavily predicated on the rapid footwork and if he's unable to step out of the way of Aldo's hands his head will be much easier to hit and again, with his chin it might not take many. That is not to say Dillashaw's head movement sucks, it doesn't at all, but the footwork makes up much more of his defensive ability and his entire offense is built around it.
For Aldo leg kicks are undoubtedly key and he has to go to them early. Also, I would like to see Aldo lean more heavily on body punches, he has a tremendous ability to chain wicked hooks to the body in upstairs-downstairs combinations and it would be nice go see him use that at any time he finds Dillashaw remotely stationary. As stated, Dillashaw's head movement is good enough that early on his chin could be an elusive target but the body will be there and sapping his gas tank is very important. Also, forward movement, TJ's pace is of itself a weapon and he likes to lead the dance. Aldo has to cut off the cage, which he's very good at, and keep Dillashaw on the back foot while anticipating the darts and shifts, creating collisions when TJ decides to move forward would be great. Move him back with feints and 1-2s, cut him off laterally with low kicks and intercepting hooks, and fight the in-and-out attacks with the straight right or check hook counter and those awesome pivots and he could take this in devastating fashion.
All in all though, I do definitely lean towards TJ. Pace and high pressure have been problems in Aldo's later career and TJ has those in spades. Also, even though he is still a beast and trouble for anybody, it cannot be forgotten that Aldo is past his prime for sure. He is not shot by any means but he is not the fighter he once was which is saying a hell of a lot about his prime because he's still impressive with a decent chance against anybody. Where TJ is in regards to prime is anybody's guess. He didn't look the best in the Sandhagen fight but Sandhagen is a hell of a fighter with a huge size advantage. Even layoff and post-EPO the pace was still there though so I think he gets it done.