Depends. Xerath is in a much better place right now than malz. His poke is incredible, and his mana sustain problems aren't too bad. both are immobile mages, but zeraths range makes up for it.
Malz is not in a great spot right now. his early game hurts something fierce because his mana problems are only solved by smart pushing your lane, which is an issue when you're an immobile mage. he has nice damage, and his 1v1 power is insane, but you have to be VERY smart about how you play him and how you team fight in order to get off your combo and not be instagibbed.
If you like the risk/reward system, malz has it. if you want a consistant, power champion, xerath has it. Honestly i would go xerath and wait for malz to show up on FTP, he's not a champ for everyone's style.