We can argue back and forth about who has been the best player, all era's confounded, even if the margin for counting Federer out is an ever receding entity. But, for a matter of comparison, let's not waste our chance of also using Nadal!
A blogger "whose name shall not be written" has published a very extensive and detailed comparison of both of their forehands. I will not post any link to the web page since TT sometimes view it as a form of advertisment or poromotion, but you may find him through google using "tennis" and "speed" as key words.
Here it begins:
The ready position
Breaking the triangle
First forward move