I like
Fiona. To me it seems youthful (not childlike, just not "older" yet), and modern, and smart.
Fenella sounds older to me, more old-fashioned, softer. It's unusual enough in my experience that I think it sounds like more of a UK name and I'd be surprised to see it on an American. I don't like the soft sounds in it. I prefer the crisper sound of
Fiona.
- mirfak