I took 3 semesters of Japanese in college, and personally it wasn't too hard. I managed to learn Katakana and Hiragana without really sitting down to learn it. The kanji is a ***** though, but if you take it slow, I can't imagine it being that bad. Sentence structure isn't terribly difficult to get down. The hardest part I had with it, was simply remembering what words meant. But you might find that easy, who knows.
I voted Japanese, but try both and see which one you like better. I think that'd better than any advice (although the advice is still nice too!)
DFO: Astenhof 35 Soulbender, DoomPig 14 (going mechanic)