The Canon 50 megapixel cameras are now available for pre order. The product is not scheduled to ship until June, but both Amazon and B&H photo have them listed. I have my pre order in for the 5Ds R. Nikon did the low pass filter experiment with the d800 and the d800e. I think the majority of landscape and fine art photographers are going to go with the 5Ds R. As far as studio and product photography goes, I am not sure what kind of situations those photographers will run into, so the 5Ds might be a better choice. Click the links below to pre order your 5D of choice.
To learn more about the 5Ds series here's an interesting look at images made with the prototype cameras.