I use both 180-degree and 45-degree viewfinders regularly. I pick the viewfinder I will mount depending on the subjects I expect to find and shoot on a particular dive.
If I am shooting subjects that are generally more macro in nature, subjects that are typically on the bottom or near the bottom and moving relatively slowly or not at all, I find the 45-degree viewfinder helps me keep the housing low to the substrate and gives me a more direct shot of the animal's face, rather that a view looking downward onto it.
But if I expect to be shooting fish or other subjects that are normally up in the water column, I find following their movements is much easier with the 180 degree finder. Especially if you want to try for vertical shots. If I were going on one of Jim Abernathy's shark trips, I would expect to do almost all of my shooting with the 180-degree finder. The faster or more frequently a subject moves, the easier it is to locate it and frame it with a 180-degree finder, which "aims" directly in your line of sight. And the eye not on the viewfinder can help you lock onto the subject in the finder.
That said, if I am shooting very small subjects at higher magnification, such as at 1:1 or greater, it can be considerably more difficult initially finding them with the 45-degree finder and there the 180-degree finder can make things faster and easier again.
I would find it very difficult to be restricted to only a 180-degree or 45-degree finder.
Fred