I had the same questions when I entered the underwater camera market.
I finally found a camera store in Dallas who's manager is an avid diver and underwater photographer. In a nutshell, here is what I learned.
The Sea and Sea/Sealife cameras are fine for entry level underwater photography. The cameras they use are usually made by Ricoh, and due their job well. However, they are usually more limited than other manufactures (RAW ability, white balance settings, effects, resolution, shooting modes, writing speed, manual modes, etc.).
Sea and Sea/Sealife are good about replacing cameras and housing if there is a flood due to a manufacturer defect. Canon, Sony, Olympus and others are pretty much say use their housings at your own risk.
You can buy a strobe, and synchronize the strobe to your Canon, Sony, Olympus, etc. camera.
In the end I decided to purchase a Canon S70 (w/ 1 gig card), Canon housing, and a Sea and Sea YS25 strobe w/fiber optic cable. The total was around $1,000, and that was comparable to the Sea and Sea/Sealife models out at the time. I made the decision because I felt the Canon gave me more control over the camera, and would make a better topside camera. I figured the slightly higher price for the Canon set up was made up for by the fact I did not need to buy another camera for high quality topside photography. I could have one camera to satisfy all my needs.
Two years later (40-50 dives), I have had no problems, and don't regret my choice.
Phew, I'm done.
I hope that helps you some.