Unlock Potential for Soleil?

unless you are planning a 4 star super merge(+10) I would say promote the one you are training, because it will ultimately have better stats

Promote the one with the best IVs, the red pool is bloated,and the chances of pulling more off banner are low, so supermerging will be hard even at 4 stars, and 5 star gives better stats than a +1 4 star.

Promote to 5 stars. Even at 5 without merges she's a fantastic sword unit. If you get mpre feathers later, than promote the 2nd for merging. Enjoy her!

Promote all soleil's and then merge her.

Soleil deserves 5-10. She can destroy all greens, most reds, and a lot of blues in her firesweep weapon and heavy blade.