I see the new tour I got in the mail came with a new SIM card. The card looks a little different than the one in my previous Tour. However, I don't know if I should keep my old SIM card, or if I should use the one that came with...
what should I do and why?
You can do either. If you put any phone numbers on your original sim card, then keep the old one. The phones on Verizon do not use the Sim card unless you are going overseas. So, unless you use your device overseas, it does not matter.