Federer or Woods.. wow, cool subject...
I am going to pick Tiger Woods... and I will tell you why.
He came out from amateur status and immediately became dominating in his sport. That was not enough for him, so what did he do? He changed his swing... dramatically. And it screwed up his game enough that people questioned if he would ever be back. And, he found his new swing and he's back even stronger than ever. The guy has dominated and intimidated for so long.
Federer, on the other hand, while great at tennis, has not been around nearly as long as Tiger and has not maintained his dominance that long. He is very intimidating at this point in his career, but can he keep it up for so long?
Also, if we were to look at it from the perspective of which one has had a greater impact on their sport, its Tiger Woods, hands down.
I am curious to know others thoughts here. The above is my opinion and I am open to other thoughts.