The first one would probably hurt my feelings more, but physical attraction is something you're aware of early on, I assumed by sexual compatibility you meant sharing fantasies and the like. Which in polyamorous relationships isn't as much as a big deal if you don't share 100% of the other's kinks, as they can find someone else for the ones that you're not into.
The second one would hurt me more as a person. I would feel worthless, used for my body, and not self-respecting enough to have tried to get what I wanted: an emotional connection. It would hurt my own image of myself and I would like me less. As I said, I'm not someone who "can do it in any order". I require an emotional connection first. Having sex is a huge leap of faith I make, a huge risk that takes a lot of trust, and doing it with someone I'm not emotionally close to only to see them go away would make me feel like, well, I called for it, really, by trusting someone I shouldn't have.
In the first case, I would still have a close friend I care for very much. In the second case, I would have nothing, except memories of things I did and now regret. I would definitely prefer the first case.
I'm not sure what you mean by "a love (potential)" though. Are we not a couple yet? Because then, it's not like I lose anything at all...
Me: 30F, straight
Seamus: My husband, 32M, straight
Dragon: My boyfriend, 29M, pansexual, married to Fox
Fox: My boyfriend, 29M, homoflexible, married to Dragon