Which champ should i buy

You should try Fizz, he is a great champ and with enough practice you can easily dominate most opponents.

already own

i want diversity in my mid champ pool

Are you accustomed to the skill-set switch that Jayce and Elise and Nidalee have? If you are, then Jayce might be a good pick, because he has decent burst and wave clear, and he can take down towers easier than other mid-laners because he benefits from AD. Also if they run away you can switch from hammer stance to cannon stance and get some last hits and maybe successfully get a kill. Xerath, in my opinion, is good when the full combo of E-W-Q then an R finisher is landed, but it sucks when you run out of mana, because once the mana runs out, thats when the damage potential ends. I own Xerath, so while it is annoying to run out of mana, he has significant poking ability with his fully charged Q, as well as wave clear, kind of similar to Veigar once he hits his mid to late game power spike.

Hope That Helped!

thanks

I'm a Xerath Main, I'd recommend him. Very fun. Solid damage, great poke, great greak poke. A slow, a stun. A long range ult that can kill a fleeing low life enemy or turn a fight you cant get to in time to your teams favor.

Has no mobility realy, but its easily balanced by his range. During teamfights the damage he can pour into the fight ans the slows/stuns he can dish out can turn it in your favor.

He deff needs mana, but his passive is great for mana sustain, I recommend getting two of the mana regen pendants to start with to build later into morellicon (whatever its called) You can lane all day if you dont go overboard, even WITH steady poke. And get Teleport, or maybe heal. Teleport more useful though. Ignite useless on him, you dont want to get that close to an enemy. Ghost OK too early on as your trying to learn him, for more escape ability.