As rsnadel suggested check the coverage map as there could be a difference in data verses voice coverage in your area. Also, while I prefer AT&T service for all of my wireless voice and data needs , if in a rural area I would also check the coverage maps for all carriers to be sure I would be getting the best signal and speeds possible for the area I would use the device most.
As far as type of device is concerned I have the USB 881 device and chose it to allow for use in both a laptop and desktop PC. Upside in addition to the multiple use factor is the ability, with a USB extension cable, to position the unit for better reception. The downside versus internal units is the obvious care that you need to take with the device protruding out of the USB port. To my knowledge there is no significant difference in performance between the devices.
You have the 30 day return policy to try it out and would only be required to pay for service used in the 30 day time frame.