re: Which is harder to do.....

It would be more difficult to return a serve "in play" than hit a baseball "in play". ( assuming hard serve and hard fastball ).

However. A tennis serve returned "in play" is considered a successful return while puttin the fastball "in play" is not a guaranteed success. Pro ball players put the ball "in play" 8 or 9 times out of 10. The best ball players will avg 3 out of 10 success rate( base hit )

So for this reason it would be more difficult for the avg joe to have a successful at bat than a successful return IMO