So the difference between an Ikelite housing and the WP-DC35 is about $100
and the cost of the adaptor is about $100
If you have the choice, do you go for the Ike?
Ordinarily I'd say go with Ikelite in a heartbeat, but there are some unusual issues going on here, so I'd say it depends on your planned use for the camera (and planned dive depths, if you exceed rec limits).
If you plan to exceed the Canon's depth rating (130'), you're definitely better off the with the Ike.
If you tend to abuse your housings, the sturdier plastic of the Ike may be a better choice.
However, if you do NOT plan to buy add on lenses (use only the camera and internal flash by itself), I think the Canon might be a better choice... the Ike diffuser is pretty pathetic and the lens tube shadow will be huge.
If you DO plan to buy add-on lenses, if you have a lens in mind, the how well the specific lens works with each of the two housings may determine your final choice... it's possible one will work better than the other, for each given lens possibility.
And finally, there are ergonomics to consider. Frankly, I'm pretty sure Canon's ergonomics beat the Ikelite's. I am very impressed with how the controls mesh with the various dials. The mode control dial work perfectly, and has good tactile response. Ditto for the front control ring knob. I actually think I like the left side placement of the Canon better than the Ikelite's front right placement. (BUT, if you have a handle and tray for external strobes, maybe it's easier to access the Ikelite knob, because all controls are on the right?)
I think the Ikelite housing is pretty useless if you actually want to use the internal strobe, even just occasionally. While I haven't taken it underwater yet, based on looking over the housing, and past experience with Canon housings, I strongly suspect the Canon housing will do MUCH better in this regard. (OTOH, if you will only ever use an external strobe with your camera, this probably doesn't matter.)
I wish was a clear answer for everyone, but I don't think there is.