I actually prefer Daisy over Peach. I'm not sure completely why. I kind of like how Daisy seems a little tougher, more tomboy like. She seems more like she could hold her own, and I really appreciate that. Plus, I generally prefer brunettes to blondes.
The thing is, Daisy should probably just remain as a costume for Peach. Even though I prefer her over Peach, I just think they are too similar to each other, and developing a unique moveset for Daisy would be tough. There's nothing really to base a new moveset on. Hell, even in the Mario sports games are they pretty much the same character, with only little differences in speed, power, etc.
They're both princesses. They both have played similar roles. They are not the same exact person, but are really close as far as their capabilities in the games they are
both featured in. I think making up a new moveset for Daisy in SSBB would be too much work that could be put towards better causes.
Besides, if you throw Daisy in there, as major as she
isn't in the Mario series, there's too much of an obligation to throw in all of the other minor characters, like Waluigi, Toad, Bowser Jr., Kamek, and anyone else. Since you can't add them all in, why start the problem at all by throwing Daisy in there as her own character?
Keep her as a costume for Peach, or nothing at all. Like I said, I have no problem with her. It would just become a potential hassle if somehow Daisy
had to be in the game.