Ratio of modems to subscribers (a modem pool of 8 mixed with 20,000 subscribers means lots of busy signals when you try to dial in)
Rates: Compare one-time start up fees. What does that include? Will they set you up with software?
Compare usage fees: Do they use a flat rate or are you charged by the hour? Consider your own usage patterns. If you will only spend 10 hours on-line each month, you don't want to go with someone that charges a flat rate of $50/month. You would be better off with a provider that charges by the hour (assuming that their hourly rate is reasonable)
Technical support: What kind of ongoing technical support can you expect?