Two Cell Service Providers & Apple IDs

In my opinion, if the phones are being used by the same person, it's generally best (there are some exceptions) to use the same Apple ID. If they are being used by different people, they should always have separate Apple IDs.

Who the carriers are for each phone doesn't figure into the equation at all.

If you have one ID, you have access to all your data and purchases on both phones. Although you can share some data and purchases, having 2 Apple IDs makes things a bit more complicated. If you want separate data and purchases, then 2 Apple IDs is better.