This comparison is very difficult because of such difference between two sports.
In tennis, a dominant player have much, much higher winning percentage, which is ~75%, whereas, in golf, anyone not named Tiger, a dominant player is winning ~12% of events entered. And because of this winning rate disparity, it's very hard to make the comparison without having both players winning percentage and their contemporary's winning percentage to see who has been more dominant in their respective sport.
The greatest golfer of all time, Jack Nicklaus, has only winning percentage of 15% (I'm being bit generous) and in majors, he has a winning percentage of ~11%. Tiger's winning percentage is nearly twice that of Jack's at 29%, and in majors, it's at 30%. The winning percentage difference between golf's GOAT player and Tiger is simply astonishing. I venture to say Federer's winning percentage isn't nearly twice as tennis' GOAT.
Federer is a great tennis player and I have no desire to diminish his achievement but he hasn't even dominated all of tennis players, Nadal has a winning record against him, not by few but by a big margin. No one in golf has head to head winning record against Tiger.
Again, it is hard to make comparison, the only similarity of two sports is, it is an individual sport, but that's where similarity ends.