I used to play _only_ Xerath (I just didn't feel like trying other champs at the time) until recently, when I spent some IP on Vel'Koz, who I liked because all his abilities were skillshots. When I tried him at top, as mid was already taken, I failed terribly, but when I called mid in another match to try Vel'Koz, my items barely changed my damage, so I didn't even need to recall for 10 minutes in game, and my poke was amazingly strong. I simply spammed QW on the enemy Kat at point-blank range, then used R to get rid of her when I harassed enough...
He's more mobile than Xerath and lets me go at an extremely close range without even worrying about my mana or HP, after rushing Athene by taking two mana regen items at first level.
His main problems were:
He can do sustained damage by poking his instant cast W, then do an extreme burst with Q, E or R. Xerath only had slow-casting bursts, and his ult is... Not ideal.
He's actually more mobile than Xerath, and can easily afford to go at melee range sometimes, while dealing the same amount of damage.
He can instant-kill the caster minions with his combo at an earlier level with less items.
Am I just not playing Xerath correctly? I make sure I'm never in a dangerous range except to proc my passive, which is probably 90% of Xerath playstyle already.