I have an unlimited text plan with xfinity mobile. Should I use iMessage or SMS

That is completely your decision.

If you use iMessage, the person at the other end must have an Apple device. But that Apple device need not be an iPhone - it may be an iPad.

If you use SMS, on the other hand, the person at the other end must have a cellular telephone.

In settings select messages and turn on send as SMS. This will send as SMS when iMessage is unavailable. I messages only go to IOS devices, are encrypted and displayed in blue bubble.