My daughter goes to both and I think My Gym is probably a tad more physical although Panda has its merits too.
At My Gym, the time for organised play is shorter, most of the time the kids just run around to take advantage of the various facilities - they have fantastic space and facilities - total time for organised play is probably less than 15 minutes and is split into different sessions. Of all the places I have taken my daughter to (Gymboree, Panda, My Gym, the Litte Gym) I think this is the best.
At Panda, the kids start off with 30 minutes free play. Their gear is not as polished as My Gym but they do have a bouncy castle which kids love. The free play is followed by 30 minutes of organised play which is not that physical but more designed to develop the children's motor skills. In a way it is a far more balanced programme, and I think Michelle, the founder and instructor, is very good.
If you are interested why not go for a trial class (which costs around HK$180) and see how you and your little one like it before you fork out on a full term. My experience is that it is easier to sign up at Panda - My Gym has a really long waiting list but if you sign up now it may still be possible to join in September (which is when their new term starts).