JJ vs TT: who is the favorite?

One pip apart, same 82%. The margin between the pairs never matters, only which one is bigger, because the smaller pair is drawing to two outs either way. Jacks and tens also share most of their straight cards, so the runouts that rescue tens through a straight often counterfeit into a chop or improve the jacks too. In a real game this all-in usually means two aggressive players ran their re-raises into each other; the tens are rarely making a mistake getting in, they are just on the wrong side of the coin that decides which pair was dealt higher.

Quick tip: Do not slow down with tens because jacks exist. Play the hand in front of you; running into a bigger pair is rare and unfixable.

All numbers are all-in preflop equity vs one opponent, computed by the Poker Shark equity engine from the full 169 by 169 hand matrix, weighted across suited and offsuit combos.