Hmm, assuming infinite stacks for Nasus' Q and Sion's E...I think Sion lives and Nasus dies.

Since both have infinite stacks, then they both have the same number of stacks.

Sion has infinity health on top of his base health (so infinity + 2171 hp at Lv. 18), along with his base armor + the armor from Thornmail.

Nasus gets (infinity + 110) damage on Siphoning Strike plus his base magic resistance.

Basically infinity + infinity cancel out leaving the 110 damage added to Siphoning Strike which can barely put a dent in Sion's base health. So Sion lives. But Nasus can't possibly have enough hp to soak the reflected magic damage even with MR (really the reflected damage is basically infinity also). Even if he had 100% lifesteal (not likely) it either triggered before the Thornmail damage hit, being effectively useless, or after the Thornmail damage hit, again useless since he's already dead.

So either way Nasus dies and Sion lives.