Speaking as a tennis player into my late HS years, I'll say that tennis, which takes great physical endurance, takes a very WEIRD kind of physical endurance. As a tennis player, one must be capable of doing essentially nothing, to a spurt of full use energy, and stop again. It's a ton of stop start, which breeds a very weird form of stamina. I'm in decent shape as a whole, I have played long games of tennis, to the tune of well over 2 hours before. But tell me to run a few miles, and I'm not gonna do so great. Tennis clearly takes stamina, but it is an odd form of stamina. My first reaction would be that tennis takes more stamina overall, however, toughness, endurance, and strength are clearly won by football players, particularly RBs, OL/DL, and TE/FB. I can't speak to the football side, since I've never played, but that's a quick take on it in my mind.
As a side note, welcome to the cafe. I hope you'll stick around and enjoy the wonderful community we have here at the Cafe.
I'm going to move this to general talk after this post, to get a better response. The football talk forum is more suited for football news, player discussion, and more directly football related discussion.