There's an open question whether the vendors are giving you as much bandwidth as the raw system can manage. If you're using the latest CDMA voodoo, you should be able to get megabits of traffic, but nobody is giving that level of service to their customers, yet.

I don't think there's any difference between using an external handset vs. an internal wireless card, in terms of performance. The benefit is having your laptop work without your celphone being connected (either via IR or Bluetooth).

In terms of dollars per bandwidth, it's hard to beat T-Mobile, since they're price seems to currently be "free if you're smart enough to realize that we didn't disable it."