I was thinking the same thing a few days ago, but couldn't resist and got both: Brand simply because he's proven effective time after time, even after his 'E' nerf ofcourse, and Xerath cause he seemed a lot of fun.
Don't regret either. Brand is easier to do damage with though... Way easier skillshots to land (meaning the enemy can't dodge em as easily as they can Xerath's Q for instance) and a lot of damage. Although I still sUck at landing Brand's Q correctly.
In 'theory' Xerath does have one of the highest AP ratios in the game though... Just look up the theorycrafting on that one. You wont be doing that from the ''artillery'' distance though, since you need to land E, Q and R R R in order to do it.