Personally, I'd go for the .46-AX. I had a twist with one of these using an APC 12.25x3.75 prop and it was a great match.

I've also owned the Super Tigre .51. I know they can be a great engine and many people love theirs. Mine was pretty temperamental though. I consider myself very good in the engine tuning department but that particular engine was a pain. You'd correct one problem only to have another. Maybe I just got a lemon.