Which Robin should I upgrade?

So they're both 5* Robins, right? The second has -Atk, +Spd, -Res, so your numbers don't add up. Anyways, you want to try to get Robin's speed as a +IV, because he really needs to not get doubled to survive. So if the second one is -Atk, +Spd, I would take that one over the first one (though what you really want is -Def/-Res alongside +Spd). Either wait for a better Robin or build the team around him to compensate for his lack of attack.

Why does your first Robin have a higher stat total than your 2nd?

Anyway, it's mostly a wash. Unless defiant speed is active, Robin cannot double most Red units or Takumi, and it still takes him 2 shots to kill off any those units regardless of 36 vs 40 atk. Tiki(Y) will take 3 shots to kill regardless of 36 vs 40 attack. The exception is Tiki(A), whom higher attack Robin will double and kill in one round while the lower attack one can't. And regardless, he'll still be a joke against most blues and greens.

If you had an ally that gave a speed buff (Lyn, Jeorge, etc), I'd actually favor the +speed one as you can still two-shot most Reds+Takumi even with the 36 atk, and you might end up getting just enough speed to double some of them.