Should I play Tennis or Swimming?

my aim is to build muscles especially biceps and shoulder muscles.
so, there is one country club that is near us : which helps building muscles faster?tennis or swimming?
do you think Michael Phelps has a 6 pack abs thanks to swimming or does he go to the gym?
Thank you for your replies :)
and if you can please explain to me which one is better?
Michael Phelps has a 6 pack abs thanks to swimming. Swimming burns a lot of calories. One of the more challenging swimming exercises is the butterfly. You have to understand that there are many different swimming exercises that burn different amounts of calories. It also depends on how fast you do them. If you could vary your workouts you will not get bored. If you do swimming and tennis as well as go to the gym you will develop muscles. However you will have to eat. Eat properly that is to fuel your body as you expend all that energy doing so many activities.
Swimming for sure no way that tennis could give you the workout that swimming does...

Swimming takes a lot of energy therefore you burn a lot of calories and it also builds muscles since you need more muscle to move in water than air(you move trough air on land)
I agree. With swimming you're using muscles in your body that tennis won't even touch on.

You can also add ankle weights and so on.
However I'm sure that Phelps goes to the gym as well. I would say swimming is more useful to bun fat and to build muscles. I mean during tennis there's always only your right or left arm and shoulder stressed. Nevertheless I would say that the mixture is the right way. Focus on swimming but other things to bring some variety.