Luiz and everyone else are absolutely right - what matters more than anything is that you get out and shoot. It doesn't matter if you have a 20k camera system and housing if you can't afford to go diving or don't know how to use it. Get a system that you are comfortable with and can get in the water. Consider your diving, your depth needs (this is where the material difference becomes important - my choice of an Aquatica over an Ikelite was because of the deep wrecks I dive when in the Great Lakes - most plastic housings (ike, sea & sea, etc) are good to 180ft or so, and the Aquatica is good to 300ft).
Working on a dive boat, I've seen people with every type of housing. We get quite a few Ike housings, old housing modified to house other cameras (20d in a DX-30/60, for example), metal housings, and even some homemade housings. What is most important is that you know how to use it. A great example is one trip in the bahamas where we had several divers with ikelite housings, and 2 divers with seacam/1DsMarkII combinations. The Ikelite/350D divers were consistently producing better pictures than the divers with the Seacam housings - this doesn't mean that the seacam is an inferior housing, just that knowing how to use your equipment is the most important thing.
That being said, if you can, get your hands on some housings, see how they feel to you, and then go diving! Whatever you chose, you'll be able to produce great images.