I prefer tennis. You have to be quick on your feet. I suspect there are more tennis courts than golf courses, so there is easier access. It is also cheaper.
Tennis requires much less equipment than golf, and its easier to find free tennis courts than free public golf courses. Tennis is less frustrating if you lose the ball or make a bad shot-- all you have to do is get another ball and start over. Golfers have to search.
id say tennis because it is much more active and dynamic. there is a close interaction with another person which makes it fun. even though you can golf with others, the competition is only with yourself... i just play solitaire when i want to do that!