I say the first one as well for a few reasons. Not just because the background seems to do better as a comparison and shows him happily wriggling through the tank, but also because the second picture, while it shows his colors better, does little of anything else.
Also, from what I've observed of past photo contests is that those with an interesting background or some interaction with something (and not just the fish itself) do better generally than pictures of just the fish.
Best of luck, he's got some coloring I've never seen before. Love it.