For low volume ground you won't get a deal on the rates, but if you ship a lot of stuff to the same zone, you can play them off each other (UPS and FedEx) to get a discount on that one zone.
If it were me, I would get both UPS and FedEx accounts and use their software. UPS has WorldShip, which is not bad and it's free. I forget what software FedEx gives you. They used to give you a system called PowerShip along with a computer, but I think you had to have 75 packages a day to qualify.
The best thing to do is call each carrier, tell them what kind of volume you expect to have, and see what they have to say.
Do you already have a customer database or an order entry system? WorldShip has some simple integration tools to help integrate their system to any database that offers an ODBC driver (which is most of them).