I would say French... for the following reason:
All of the conversational "sounds" in spanish are found in english. There's a nasal quality to mesoamerican spanish, but that's found in english as well. The rolled R is a tongue trick... but it's a sound readily heard by English speakers... it just takes practice to duplicate.
French, however, has sounds that are NOT in English. If the brain doesn't learn to listen to/for them she won't be able to hear them (same with japanese). What's interesting about French, is that the sounds one hears in French... "translate" to Spanish nasal sounds, but don't translate in reverse. They also translate fairly easily into the "choking on a moth" Dutch sounds... but Dutch doesn't translate back to french.
I say this as someone who (american, native english speaker) grew up in Japan (and used to be fluent in Japanese). I can hear asian sounds that most people can't ... and I also speak spanish (badly, but I can understand), Italian, German, & a little Swedish/Norweigan. But I CAN'T hear French. Not all of it. Being in France (literally, not figuratively) gives me a headache. It's like being in a badly dubbed kung-fu movie. People's mouths are moving but there's no sound coming out during certain parts. Whenever I'm in France I get tension headaches, because my brain is TRYING to hear something it doesn't have the neural pathways for. I can understand my SISTER speaking French, and therefore know there are SUPPOSED to be sounds there... because she speaks with an American accent. So she's not getting the sounds "right"... but it's better than silence.
Every language above except for French I was exposed to as a child. I've got the neural pathways. But I didn't hear French, at ALL (except for in dance class), until I was an adult. My sister lives in France. When we meet up... we instead split the difference (my brother lives in Germany) and we meet up in Switzerland. I love the french countryside and riveara, Paris is okay... but I avoid the country like the plague... because it hurts my head too much to be there.
((BTW... We're teaching kiddo Latin -Minimus http://www.minimus-etc.co.uk/, but haven't yet decided on a curriculum for spoken language... Are you using Rosetta or PowerSpeak or something else, and how do you like them?))
R. ((HS'ing our 7yo, hence the recommendation request :))