I never had a cellular iPad before. Those carrier plan, they are month to month right? No contract and can cancel anytime? Can I buy a Verizon iPad and still use AT&T and Tmobile sim card? And vice versa? or Verizon iPad on CDMA only? Can I use prepaid sim cards like Straight Talk or AT&T GO? How do I go about using those in the iPad? What is the best data plan and which carrier?