Quan Chi is totally naturally prone to manipulation. The dude is a hell demon, after all. With all his iniquities, though, you have to hand it to him that he did was of some notable use to combatants, too. E.g. he did arrange that Scorpion get killed but he also transformed him from a mere human warrior, to a powerful spectre, whom only the elite MK combatants could take in a fight; he did fool Shinnok with the quasi-amulet, but he also arranged his expansion from his Netherrealm confines. That said, you could probably expect some sort of great, supernatural service from him as well; the price would be dear, of course, but hey, so would be the reward.
And also, who's to say Shang Tsung wouldn't betray you, too? He just happens to serve a master whom he fears more, than Quan Chi does his, but if he were more brave and powerful, he'd likely sabotage him too (like, in the long run, he did in MKDA).
As to the question, that is a tough one. For more security, I'd get enlisted under Shang Tsung; for more power and knowledge I'd look up Quan Chi. I would likely end up with Tsung, though, since his level of power would fulfil me fine enough, and I'd take a fellow human, even a malevolent one, over an unpredictable demon any day. Shang Tsung, even as of now, should have at least some minimal care of his mother realm, while Quan Chi, literally, couldn't care less if the Earth perished to its last atom.
This question has yet another level, though: whomever of sorcerers I elected, I'd almost inevitably fall under the command of either Shao Kahn or Shinnok. So it then boils down to whom of THESE two I'd serve more gladly. Hah !
