I have a friend who has a TERRIBLE bum knee and she has been using her eliptical machine everyday for the last 6 months with no problem.
I haven't tried a glider, but I have tried the eliptical machine and had a very strange problem with it....it made the balls of my feet hurt!!!
All I can figure is that that is the one part of your foot that stays planted the whole time....you sort of lift the pressure off your heel as you complete the circle. So my advice is pay for one day at a gym somewhere that has both and do a whole work out on each one to see which fits you best!