Xerath if you want more powerful much more reliable basic abilities. Ziggs takes it with his explosive ultimate and a little more utility.
Xerath Q zaps an entire row at once. Ziggs Q is very very avoidable from range; it's just a bouncing little pellet spec. Xerath has more direct burst compared to Ziggs on his W/E. But yeah compare ults then Ziggs could take it in a blink of an eye; however Xerath ult is tons of fun for cleanup.
Xerath has to and wants to position further back than Ziggs.
Ziggs is cute but for full tryhardedness go Xerath. I find Ziggs isn't dependable at all if your lane opponent and targets are at range and can sidestep his Q's. Unless they're melee, then they have to last hit creeps and eat bombs. Perhaps Ziggs is better at disengaging himself from THOSE champs mid lane than Xerath is.
IF you're doing well as Ziggs though he destroys bases super fast. So assuming equal success he closes games out easier.
So technically Ziggs can fit better into CC wombos, but Xerath is more consistent depending on your own skillshot accuracy. With Ziggs you would honestly be more reliant on team CC.