Choosing a Buddhist partner ain't beneficial to spreading Dhamma?
Choose a person with compatible generosity, compatible morality, compatible faith and compatible wisdom. That means even among Buddhists, you still have to filter out the potential spouses. 

Marrying a person of a different religion could be highly problematic if you are a practicing Buddhist. I have seen some people make it work, but those are mostly people who are simply Buddhist by birth and not interested in it much and don't care about what religion their children pick. So either you have to be that way or the other person have to be that way to make it work.