My opinion is go for whatever feels natural. And like, people hooking up within a tiny circle of other people sharing their trauma is a thing. Developing a romance is a lot easier with other player characters because of the sheer amount of time and experiences the characters share by default.

Romancing a guest character is harder because it would be either very brief, or the guest would get prefetential treatment one way or the other. That can be awkward. Romancing an NPC can be awkward because especially when the NPC is enough of a fighter to go toe to toe with an ancient red dragon twice, finding reasons to conceivably keep them out of the narrative without it feeling forced can become awkward. Otherwise, the risk of DM!PCs happens.

I dunno, I personally will take PC/PC over all the other options any day, but that’s just because I thirst for all of the content. But the logistical issues are a thing!