Is there an advantage to purchase an unlocked Verizon iPhone?

Hi

We have two Verizon iPhone 5 phones that are eligible for renewal. Is there an advantage to purchase an unlocked Verizon phone and thus not be under contract?

Would I be able to use an unlocked Verizon phone on T-Mobile or AT&T. should I decide to leave?

We have a shared everything Plan on Verizon and we are month to month with regard to our iPhones.


Thanks

Jay