Whatever you wear, make sure it is comfortable enough for you to easily get out of your seat every couple of hours and walk around for a few minutes.
Long plane flights are known to cause blood clots to form in the legs which can cause heart attacks or strokes, and THIS IS NOT ONLY FOR THE ELDERLY OR THOSE WITH MEDICAL CONDITIONS.
I recently read of a healthy 28 year old young woman who after a long flight had a stroke!
With the OK of your doctor, you may also want to take a baby aspirin once or twice during the flight (for the same reason).