The size of the watch on your own wrist usually answers the question. The answer for me is 5164A. Personally I wouldn't want my watch to be larger. If you can 'handle' them both you can consider the functions. Do you prefer a Chrono or a Dual Time?
Also; Do you prefer a cleaner dial? Or one with an extra color on the dial?
Hope this helps! Aquanauts are really great watches. You won't regret it. I think they both look very good.
Best,
H