Do what will keep you going.... Its all about getting your heatrate up on a consistent basis. Your heart doesn't care if you are on a treadmill, bike or having sex......ok, maybe having sex, but the point is to keep doing whatever it is you like. If that means mixing it up, then switch activities.
I prefer kickboxing/sparring.
--JEFF, 36, 6'3", 230LBS
Turn off the computer, Get in the gym, Shut-up and Lift!
Some people say treadmills and running are real bad for your knees and elipticals are much better because there isn't as much impact. Personally I prefer the treadmil, elipticals burns my legs out to quick before I can even break a sweat.