Hi Julie! I've never tried a gazelle, so unfortunately I can't give you any advice about it.
As for bikes, I personally find them much less challenging than ellipticals. IMO, bikes are an easier workout and my fear is that you might be disappointed in the level of difficulty. Have you had a chance to compare a bike to your elliptical?
You say that you've maxed out your elliptical's resistance. Have you tried increasing intensity in other ways than resistance? Speed and/or intervals, for example? I find that I can get my heart rate higher by
lowering the intensity and going faster - like 180 - 200+ RPM. Since you're already doing strength training for your legs, you don't need to try to build muscle with high resistance. Have you tried FLYING on your elliptical to see if it gives you a better workout?
BTW, I don't think it's a big deal to shake up cardio the same way we change up our weight routines. As long as you're getting your heart rate into your target zone, you're getting the calorie burning benefits of cardio exercise. I did the same amount of cardio on the same machine every day for a year and lost 122 pounds, so I think the same old routine worked OK for me.