Tennis takes more athleticism. Tennis player run around constantly, and need to make quick adjustments and movements on a more consistent basis than baseball players. They are far more active than baseball players... you will find a 250-pound power hitter in baseball, but a 250-pound tennis player would probably not be very successful.
As far as talent... that one I lean towards baseball. Unless you've done it yourself, most people have no idea at all how hard it is to hit a 90 mph fastball with a 3-inch wooden bat, much less a curveball that breaks 5-6 inches in the last 20 feet before it reaches the hitter. Tennis players receive serves that are at a higher speed... but the surface area of the tennis racket allows for more leeway when receiving a serve. If you're 3 millimeters off on your contact with a baseball, the ball goes foul.
As someone who's played both competitively in his younger days... tennis definitely has the athleticism element in it's favor... but I do think there's more room for error in tennis and players without excessive talent are able to compete, whereas natural ability is more required to have any success at all in baseball.