I'm with AT&T and I currently pay $69.99 for voice (700 minutes), $30 for unlimited text and $40 for data (2GB and 250MB plans). So roughly about $75/month per line. Not a bad deal imo.
In the meantime, check this out. It delineates and goes through all three offerings.
That's an awesome comparo chart. It should be a sticky in this forum!
If you are considering Sprint. Check out the link below which gives you instructions on accessing Sprint's Everything Plus Referral Program. Click on the "Check it out here" link and enter the guys e-mail address and ID; I think he was the guy who started the program.
I used it a year ago when I switch from ATT to Sprint. Link looks old, but it still works, they have the IPhone 4 on the list of phones, I'm guessing the 4S will be there tomorrow.
http://mcguireslaw.com/2008/07/16/psst-have-you-heard-about-everything-plus/
Sprint dings you with an additional $30/month to tether.