The Ikelite housing will allow you to run a wired strobe, the Fantasea won't. The advantage of a wired strobe is that you don't wear down the battery of the camera in the housing. I'm not sure what Interceptor121 means by "Top of the Line" that's not true, you could use the Ikelite DS51 which is not a top of the line strobe. Ikelite also sells TTL Cables for Sea&Sea and other strobes. The housing came with a diffuser if you use the internal strobe to help solve the shadow problem, but in my opinion the internal strobes on all of these cameras are too weak to be of much use underwater in any case.
The ikelite housing also has the benefit/detriment of being clear. Benefit: You can see the o-ring and know that it's sealed well, I've have once or twice gotten something out of whack putting it together and looking at the o-ring could see it wasn't sealed and fixed the problem before the dive. Detriment: Some people would say it's not as durable, I've had the G9 one and now the G15. The G9 held up very very well, and I know several professional photographers using the IKELITE DSLR housings that have many 100's of dives on them with no issues...