My criteria was price, available lines, and support. Finally, there is support. I've had mixed experience with Global's technical support folks in the five months that I've been using them, but that could be a function of rapid growth as well. Basically a matter of drivers and configuration. Some service providers offer both methods, some don't.
Some providers claim one method is faster or more reliable, some claim the other.
Global just offers SLIP access for now. First off, they limit the types of software applications that you can run. Second, you'll end up learning Unix commands whether you want to or not. And finally, moving files from the Internet to your computer is more of a chore than with SLIP or PPP access. However, Chameleon has done a great job of simplifying things.