hey gymgirl,
You probably know more about this than i do, since you've been at this whole fitness thing alot longer than me, but i can post what's been my experience with the cardio machines:
Ellipitical/cross trainer: gets my heart rate up more (alot more). I use it for high sustained intensity days, hiit days, and days I want to vary the muscles in my cardio (5 min forward, 5 backward, no hands, hold on and sit back (feel the burn in glutes & thighs!), lean forward and use arms only, etc etc etc
Bike: moderately elevates HR, I find it difficult to get my HR above 155-160 no matter how hard I ride. But, it focuses on the legs/quads mostly. I use the bike for endurance cardio days, day-after-legs to help soreness, and to help hit that inside part of the quad that makes the bump down near your kneecap- bike days are the only time I feel that part gets significantly worked.
At least once a week I use all the cardio machines in the same cardio workout- jump from one to the other every 5-10 minutes (with no rest in between), and feel the difference. Going from backwards elliptical to jogging on a treadmill makes my calves get all a-quiver
Hope this helps! I follow a cardio plan to do 1 strength day, 1 interval day, 1 endurance day, 1 maintenance day each week for maximum results (designed for bike racers, and passed on to me by my father, the Machine, who trains with racers for fun).
Bike or ellipitical / cross trainer
