Personally I prefer to pre-pay, it kinda keeps the possibilty of charges going through the roof. The benefit of post-paying is that unless your CC transaction is declined then you shouldn't see a break in your campaigns.
HTH
I am wondering how adsense bills advertisers? Is it an auto-bill or do you have to prepay? And do they just accept credit cards?
Tomld2 - Below, I've included a few links from the AdWords FAQ, which should give you the info you need. BTW, I am assuming you actually mean AdWords, rather than AdSense. If you did mean AdSense, well, that is a whole different story!
When do you bill?
[adwords.google.com...]
What are my payment options?
[adwords.google.com...]
Account Fees and Payment Options
[adwords.google.com...]
AWA