Which Soleil should I use?

I guess it really depends on the build, but I think, generally speaking, a speed boon is probably her most preferred boon. She has high attack anyway, even with neutral attack IV, so she could use as much speed as she can get to increase her doubling potential.

It depends on how you want to build her. For example, I use a Brave Sword on her, so +ATK is best. If you plan to use Firesweep, then +SPD will help her double where she needs it.