It's funny Ibbasquish...I have the same problem...I can do cardio on a treadmill just fine, but doing cardio on an elliptical exhausts me within minutes. I thought it was just me.
I can also report that an elliptical hurts my knees, where a treadmill does not. For me at least, I think that a treadmill conforms to my body's natural rhythm of motion, and an elliptical doesn't.
To the original poster: As others have wisely cautioned -- be sure to try before you buy! You may find that you prefer the gentle bobbing motion of an eliptical more than the rather one-dimensional treadmill experience.
Cheers,
~ G.