Marco -
One of the problems of trying to find truly unbiased help in this category is that many shooters are brand loyal. It's an expensive prospect to think about finding someone who has good experience with two diverse brands - so you may end up comparing apples to elephants.
Many have started out with one brand or the other and then switched over to the other. BUT the models they used were probably quite different.
I guess I haven't met many who have shot with both, say the Canon 20D and the Nikon D70 - yes, many have tried them both, but not many have really gotten to know each camera by using each extensively; the costs in having two complete and entirely different systems aren't worth it. It's still a matter of personal preference.
On the up side, I doubt you can go too wrong with either choice - just do your research to make sure you are choosing lenses and housings that will allow you to do what you want to do.