One reason to work out a little longer is because of the fat burning equation. This tells us that during the first 19 mins. of aerobic exercise you burn mainly carbs that you've eaten (glycogen). After 20 mins. you begin to use fat as an energy source and hence begin fat burning. Think about this next time you stop your cardio after 20 mins.
Don't worry about figuring out your target zone. Every heart monitor comes with a simple chart so you don't need to do any math. Finally you can spend about $40 or $400 and the difference is mainly bells and whistles. The cheaper ones are also easier to set up and get going.