I had a USB Mobile internet card from both China Unicom and China Mobile.
China Mobile is more expensive when you are buying the equipment and set up, however, in this case you really do get what you pay for. China Mobile reached most places even when I was traveling and it was about 2 times faster.
Unicom was about 300 RMB cheaper but even in Beijing there were many places I couldn't get service.
Having said that, it has been about a year since I have used either and perhaps prices have changed and services improved.
Concerning, your SIM card, if you don't care about what number you get or losing your phone, you can always get a card for 50 - 100 RMB that already has credit on the card. You can just toss the card or give it to a friend when you leave Nanjing.
However, if you lose your phone you can't regain the number nor the credit stored in your phone.
