I like
Belle on its own, as both a first and middle name. I prefer it to Bell.
I have never met a
Belle, nor do I find it to be a common middle name. However, I do think that right now there are too many Isabellas, Isabelles, Annabels, Bellas, etc for it to make a real impact.
Belle over
Bella, any day.
Just
Belle- I'm not really a fan of
Isabelle, but I do like
Isabel.
Of those choices, I'd pick
Belle.