Papyrus is more fun to be around plus he knows how to cook Spaghetti need I saw more?
I'm gonna go with papyrus on this one because even though sans is baller, papyrus isn't a lazybones and a slacker like sans. If it wasn't for sans's gaster blasters and teleportation, he would be an easy 1 shot kill. And only at the end of the game is when he realizes "S**T THIS HUMAN IS EVIL!!!!!!1!!!11!" (Genocide specific)
If you pick the younger brother, YOU'RE GONNA HAVE A BAD TIME.
he is a baller
sans he is the main boss but papyrus is 1shot 1kill sans has gaster blaster but papyrus has only a skeleton papy's sagetti groses!!!!!!!!