Unlocked or locked iPhone - which is better? - 1

I'm looking into buying an iPhone 4s. I don't want a contract (don't send that many texts/use mainly Wi-Fi/etc), and you can find these products far cheaper on eBay than in phone shops.

However, i'm unsure as to which is the best buy: one that is already assigned to a network, or one that is 'unlocked'?

Does anyone know the pros and cons to either of them?

Any advice or information would be much appreciated.

Thank you.

Added (1). Also, what happens if i buy an unlocked iPhone? Where do i buy a sim card from? How much do sim cards cost? Once i buy once, do i just insert it, or do i have to register with the netwrok? How easy/difficult a process is it?

Basically unlock phone are better because if you ever decide to connect your phone you can use any carrier in the world, where as lock phones you can only use that one carrier