I'm using T-Mobile

1) I was wondering if you had wifi and your in a hotspot connected through your cell phone and your use aim to either idle or im people does it charge or is it free?

2)Another thing is if you have unlimited text,im,picture,video messaging does it charge to idle or message on aim?

extra less relavent question:
why do people prefer the 3g over wifi, i know wifi doesn't work everywhere but it's free right? To take advantage of 3g don't you need a internet plan? or pay more separate fees, like 15 cents here 10, 5 cents there....etc..

btw i don't have a wifi phone or the unlimited messaging, just want to know for future references, and will help me make some decisions with phones and plans and stuff

thanks in advance.