Just my two cents. Housings are just the start. You will/may also need extensions for the specific lens you use and ports or domes as well. Depending on which lens you want to use on the you may also need some other parts.
Just as Nikon and Canon have different lens mounts underwater housings have different mounts so not all extensions/ports will fit on all housings. Some folks think the Ikelite housing limits the choice of ports compared to some of the other housings.
This is not to say the Ikelite housing is not a fine product with some real advantages; just that some other choices may offer different advantages.