Both strong situational picks. Both hard to play well (Syndra harder). Syndra is less team comp dependent. Both have pretty unusual feels to them (Syndra's Q and W having no cast time makes a lot more difference than you would think, and Xerath is of course way more of a siege tank than anything else you can find in the game).Pretty hard to guess which one you'd find more fun.
FACT: Four out of five people are crushed to death by giant diamonds every day.
They're both pretty good, and fill similar roles in fights. Syndra can move whilst casting so she's quite fluid, while Xerath has to root himself but he gains a movespeed steroid which provides the mobility he needs. I'd probably say Syndra's slightly better because dat AoE stun, but you have to learn how to use it effectively, otherwise Xerath is a safer pick since he doesn't have to get quite as close to be effective.