It depends why you want to change. What is the main reason motivating your question ?
What don't you like about your bard/ cleric ?
What do you like about it ?
And you should ask yourself the main advantages of a wiz compared to cleric.
I guess we can say the attack. More different attack spells. Kills faster. Maybe you like better the look of robes who knows ^^
But they also die fast, 2H staff and robe equip cost a fortune, they have the biggest aggro so you need basically to grind in party or ..... you'll die alot.
Clerics wear light armor, they have a lot of usefull buffs when it's your main class, they can tank (not like a warrior but lot more then a wizz for sure). Very usefull in party.
They have poor attack (with only 3 different attacks, one is an absolute damage sucks a little, the other one takes 95% of your HP ... and trial/ justice cross is not very strong).
Maybe other people could add some advantages/ disavatadges about each class.
You must select in function of what you prefer.
The important here is not to be the strongest on the server, but to have fun with your character. To me playing a MMORPG ... it says everything ^^ it's to play with other, have fun, make friends. Of course we all love to be strong and kick ass lol but it won't replace everyday simple fun of playing.