Neither is best for all photographers. That's why there are plenty of pros using Pentax, Sony, Panasonic, and Leica.
What's better Ford or Ferrari. Ferrari of course, right? Well, I'm a pig farmer so how in the world am I going to use a $250,000 Ferrari on my muddy farm? the $250,000 Ferrari is a useless brick, and I'd be better off with an $800 used Ford F-150. While this is a rather silly and extreme example, this still holds true for camera gear. Cameras are just tools. Based on your needs and budget, one will better for you while not so great for someone else.
Today's DSLRs are vastly better than the professional gear used in the film days. There is truly no such thing as a bad DSLR, just one that is better for your needs.
To take your question a step further, why just Canon or Nikon. Do you know that in the non-pro world, Canon and Nikons get outperformed by Pentax and Sony? It's only in the professional market that Canon and Nikon stand alone, but even at that, professionals are starting to migrate away from the big and heavy DSLRs to Sony's mirrorless Alpha series of cameras.
Again, there's no one best brand otherwise all pros who use their gear to pay the mortgage and put feed their families would all be using one model from one brand. To show you that it's the photographer and not the gear, here's an amazing photographer using Pentax: http://www.billgekas.com/