In your climate, I'd go with the amaranth. Quinoa doesn't like it hot and humid from what I've read. Last time I grew it here was 2009, which was an exceptionally cool and dry summer and it thrived. Amaranth, OTOH, doesn't really seem to care, it always thrives.

If you're feeding the foliage to rabbits, I'd also say go with the amaranth. It just produces a lot more foliage than does quinoa, which has a typical goosefoot family type leaf maybe 2 inches long, and the leaves get progressively smaller as the plant bolts to seed, so that the ones up on the seed stalks are tiny.