Neither.
Viola makes me think of "violent" and especially "violar" which is Spanish for "to violate" and if I'm not mistaken also for "to rape". I really dislike it. I know one
Viola who would be in her very late 20s now. Her sister is named
Jasmin. I also know a Ronja-Viola who is in her teens (RON-ya). Both are from Germany. I've never met an American
Viola. I like
Violet a lot better. For some reason it doesn't make me think of "violent" even though it's closer to the word.
I never really liked
Fiona. It seems to be such a cliché redhead name. I have red hair myself and love it, but it just annoys me because it seems to me that whenever there's a red haired character in a book or movie there's a 20 percent chance that she is named
Fiona. As in Shrek or that book "The Keeper" (I think that's what it's called). I never really understood its appeal.
If I had to choose...I dislike them about the same. I guess
Fiona.