I like
Rhiannon more because if there's an alternative to ending a name with an "ah" sound, I'll usually take it. (Even though I like a lot of "ah" names, I can get picky about it.)
I don't like either very much at all, though, because
Rhiannon has turned into that token Welsh name that people use when they think they're being all creative and... well, Welsh. :-/ Kind of like how
Aidan is "sooooooo Irish" and whatnot.