I think it depends what you're looking for/to get out of the relationship
If you really just want boning/companionship, theres nothing wrong with dating multiple people/open relationships, as long as all parties are aware/okay with it.
While I'm half-way in the camp that says monogamy isn't natural and marriage isn't really needed, I think one on one relationships are fantastic and can be really beneficial on a mental/emotional level, provided they are healthy and such.
I dont know the girls so I can't recomend one or the other, you know them the best though. I dont think there would be anything wrong with exploring your options as long as the first girl didn't care
Grills and relationships are overrated (still bitter over my breakup, but im in a much better place now)