Sitting is not good for your posture unless you have the right seat and posture, which I am extremely doubtful of. When I took Alexander Technique at my conservatory they taught us about seatings in Orchestra and how important it is to adjust your seat correctly. I also learned from that class most people can't even get into a seat properly, and most people do not sit correctly.
LYING FLAT ON THE FLOOR is the best thing you can do for your posture...Some musicians play immensely better sitting (Itzak Perlman) than standing but a majority I'm quite sure are better at playing while standing (you feel your legs and the ground more). Just saying Kinect standing does nothing bad for your posture unless your posture itself was already messed up.
As for sitting I would prefer Forza while sitting immensely and I hope MSFT addresses this issue for games that could be played on Kinect while seated.
