Only difference is size to save space in your phone/tablet.
I dont think there will be speed difference because you can cut the normal sim to use as micro sim.
I mean the 3G data speeds. I am using a normal sim as a micro sim as of now and get around 2 to 3 mbps on my iPhone 4S.
I googled it and found people claiming that microsim provides better 3G Data speeds so just wanted to confirm.
i got my regular airtel sim cut to fit in the one x. have not noticed any issues. it works just like any other sim. airtel wanted a day to deliver me a microsim. so i decided to get my old sim cut from the market.
There is no difference between micro sim and normal sim expact dimensions. Apple came with microsim in order to reduce size of sim tray. And that means reducing thickness of iPhone. Both sim possess same properties.