For myself, I would agree with James above. However, the optimal choice depends on your goals and style of photography.
A strobe gives you several advantages.
The first advantage is restoration of natural color. As you descend in the water column, the longer wave lengths of light from the sun are absorbed. Red light and therefore red colors are gone at 15'. Orange, yellow, green and blue follow in that order. If you are down about 80', it is a blue world. Now I have taken photos of fish against what appeared to be a muddy grey background. When I looked at the photo later, the background was actually a garish blend of red and orange. So if you are looking for "natural" colors, you will want a strobe. Remember if you are shooting in shallow water of 30' or less, the loss of color may not be that critical to you.
The second advantage of a strobe is the fact that you just have more light. This allows higher F stops and faster shutterspeeds.
The third advantage of a strobe is the fact that the strobe's duration is very short. Even with a moderate shutter speed, if most of the light is from the strobe, you can get some stop action just from it.
The fourth advantage is backscatter. If you have the strobe light coming in a from an angle illuminating your subject without illuminating the intervening water column between your camera sensor and subject, the nasty stuff in the water will be nearly invisible. Done just right, strobes can make water look clearer than it really is.
So there are quite a few advantages for strobes.
What are the advantages for wide angle? The closer you get the better. The more water between you and your subject, the worse it is. If you are taking photos of large subjects: landscapes, wrecks, divers and big fish, unless you have a wide angle, you just can not get them into the field of view unless you are pretty far away. "Far away" in underwater photography terms is anything greater than 5". I have a Tokina 10-17 fisheye for my DSLR. I can get a shot of a diver at less than 3' and that really helps.
If you want vibrant colors at depth and you shoot primarily close ups of smaller critters, the strobe is the way to go. If you don't care so much about the colors and you want to take photos of landscapes and large subjects, well the wide angle is the route to go.
It also depends on just how picky you are about your photos. There are plenty of people around who just want snap shots who are perfectly happy with point and shoot cameras without strobe or wide angle. If you want more than that, you will probably want to get both eventually.
