I wisely started keeping a scharp eye on my heartrate during today's cardio workout, which was steady @ 75% of max. for 45mins. My question is: Is there a difference btwn. being on level 4 or level 5 on the bike (for instance) but maintaining the same heartrate? Basically if I were to spin faster at Level 4 to maintain the same heartrate as I would if I spun slower on 5...it's less effort in some ways, would it be just as beneficial?

You'll actually do about the same amount of work - speaking in terms of the power you actually apply to the pedals. There are a lot of other variables I could go into (I'm a cyclist, so I know a good bit about pedaling), but they won't really make a difference for your situation.

It is very surprising but the "human machine" works pretty similar to a regular car engine in this regard.

As cars achive their top horsepower at high RPMs, so do humans.

There is a practical limit on how fast you can pedal. This changes from person to person and mostly with experience level. A typical bicycle newbie would probaby pedal at around 75 cadence confortably. Most road bike guys, who are somewhat experienced push around 90-110 cadence. Track pros can maintain 140 without a problem.

The power that you put in the pedals is linearly proportional to the force you are pushing on the pedals multiplied with the speed you are turning the crank around. So if you can maintain a higher cadence, you are better off in terms of power output. (Normally the increased wind and rolling resistence is what makes you not go arbitrarily fast :-) )

So to get to the real answer, it all depends on what your "style" is. I myself tend to feel the best around 100-105 cadence but if you watch a bike race, you will see hummindbird like riders and also big gear pushers, who turn huge gears at 80. I would basically say that since you are doing cardio, do it at the cadence that feels the best.

[Edited : track != Trek :p ]