I'm not too familiar with the inner workings and configuration differences between using a cellular phone versus a PC data card.
However, i do support mobile users who use VZW PC AirCards and have a little knowledge about them. The PC data cards tend to operate at slower download / upload speeds than a wireless AP or residential gateway. Why this is i have no idea (i assume that they are using the same radio waves, different frequencies however), but i think its because the WWAN infrastructure is not as prevalent or efficient. Cellular signal repeater infrastructure now has a greater coverage area than the WWAN infrastructure because cell phones are such a huge commodity and more people have a cell / PDA than those who have a laptop and need wireless data access.
One advantage that i can think of off the top of my head regarding a cellular phone as a mobile modem versus a PC data card is that because the WWAN infrastructure is not (yet) as prevalent, you can be in a coverage location where you have 100% signal strength on your cellular phone, but the PC data card is unable to get a strong WWAN signal. Because of this, you might be in a location where your cellular phone would be the logical choice to connect whereas your data card may not even be able to connect.
Edited by Cyb3r_Ninj@, 19 December 2007 - 01:28 PM.