It's a strange feature of PD that at the same time you can be both good and bad at what appears at first sight as similar activities. In my case, it can take over a minute to move the cursor to the other side of the computer screen, whereas I can move very quickly to catch a ball.
I think any reasonable amount of exercise is good for PwP. Both tennis and table tennis have a lot to recommend them, because they involve a wide variety of movements. You may find that to start with more exercise requires more medication, especially levodopa. But, in the long run, your increased fitness will leave you better off. There is some research evidence that exercise slows the progression of PD. At the very least it improves your mood and gives you the skills needed in every day life.
My approach is to try it and see what happens.
A final point, I would not beat myself up, if I cannot win all the time: sometimes the Parkinson's will make you uncompetitive.