Hi Colin, the only person to answer this question is you. What kind of images do you like? I like taking images of very small things, so that means I usually have a macro lens with me. But maybe you prefer scenic shots of reefs.
A macro lens can not be used for wide angle. You can take images from something far away, but then you wont get as much light on it, or even none at all. Think of it as a thin beam coming from your lens.
A wide angle lens can be used close up. You can take images of things only inches away from your dome. But you'll still have relatively large things filling your frame, like for instance a lobster, or a squid or something. Nowhere near what a macro lens can do.
A macro lens is usually somewhat easier to use, but a wide angle lens will show others what it is you're seeing underwater. Pick one, im sure you'll be happy with it. Then quickly save up for the other one 
Cor