Let's be honest and admit that bikes are popular in part because they're not as demanding, especially recumbent bikes where you're in effect lying back resting many of the body's larger hip/leg muscles that elliptical and stairmachines enlist rigorously. Not to say a bike can't be effective, an upright can be if used intensely (Spinning classes: high intensity interval training using upright bikes), but if all things are equal re: intensity, they're not as good, particularly recumbents that are using considerably less musculature. Unless used intensely or as a secondary exercise, bikes are a pleasant aerobic activity that by themselves are ok.
That said, also factor in what you'll actually use on a daily basis. I'm going to try using a trampoline as a secondary exercise to complement the stairmaster.
The numbness factor can supposedly be avoided with the use of one of the special seats..