Accessing 3G internet via USB data card is a lot more convenient and easy.All you need is the APN.So spending money on 3G data card can not be called wastage.Moreover,most of the mobiles these days have 3G upto 3.6 Mpbs,so if you buy a HSDPA USB modem with speeds upto 7.2 Mbps it will be future compatible.This will be all that you will need for quite a long time to come to access all modes of internet including GPRS,Edge,3G,HSDPA.So far as the difference in accessing 3G via mobile and datacard is concerned I have no idea but I think the speed will be more via 3G USB modem.
