Ok, so I've seen several good arguments. I'll run through my responses pulled from my ass to some of the major arguments I've seen repeated. Correct me if I'm wrong, and I'd love to see your counter arguments. I'm slightly inclined towards Sans, but that's just my bias.
One huge argument for Undyne is that Sans' stamina is low. After several attacks, Sans is shown to tire out. But he does not slow either, so his attacks continue at the same pace. He falls asleep during his special attack because he is not doing anything. However, it is stated in game that Sans only uses this attack because he knows Chara can reset, so he knows all his attacks are useless. Therefore he would never use this special attack. Also, during the attack, if you attempt to move the bullet box before he falls asleep, he will teleport you back to the center, and, correct me if I'm wrong, starts his sleeping animation all over again. You can do this infinitely, and he will never fall asleep. Undyne, since she cannot game-break, cannot move the bullet box anyways, and so sans could sleep as much as he needed and rejoin the fight.
Another is Undyne's Green Soul Mode. First of all, at no point in the game does it say that soul modes can strictly affect monsters, but for continuity's sake, let's say it can. There is a small portion of space where the sprite of undyne's "spear wave" does not reach on the bullet box. I believe, but have not ascertained for myself, that you still change if you are in that space, so you do not strictly have to be hit by the wave. However, we can also assume that there is a range, since otherwise undyne would freeze you while you where running. Since Sans can attack from a distance, he does not strictly have to be in the range in order to fight, although neither does Undyne.
Some people have said that Undyne would not give sans the spear to block with. But then why would she give Frisk/Chara a spear in genocide run? She clearly knows the threat they pose, and if she could she would not give them a spear. Thus my theory that the spear comes along with the soul mode no matter what.
Sans is said to be able to use shortcuts, which seem to be similar to teleporting. However, this also implies that he can only use those shortcuts when there is one available. This means he can't just go teleporting away whenever he wants. However, he can teleport Undyne, as shown in his "special attack", so one argument that I find hilarious is to teleport Undyne straight to hotland, or even just into lava. Unreasonableness aside, Sans could, him being very intelligent, teleport Undyne in front of her attacks to force her to cancel them.
Sans, attacking first, could technically shoot gasterblasters at undyne continuously until she died, 32 damage per second at 32 fps would actually make undyne last about 12 minutes. Using the average stated above my some person or other of a sans fight lasting 10 minutes, this only requires 2 more minutes than the average. Sans can attack during others' turn, and it is possible that he could use a gasterblaster for this as well, or just a bone at 32 damage per second. I'm not sure about this next part, but if sans' attacks are cummulative (cumulative? cumulatative?) then a ring of gaster blasters which is about 22, would be 22*32 at 704. that would be 32-3 seconds long, which is quite short.
sorry for the long post.
