I agree that belly heat is easier to use because you can more effectively control many cages with one controller. Belly heat does not always provide the best solution for certain applications however. It really depends on the type of animals you are housing. But anyway, enough of that.
I use heat rope alot. I like it better than Flexwatt for undercage heat in many applications. I use it for rosy boas up to big Colombian boas. The benefit is that you gently heat as much or as little of the cage floor as needed depending on how you pattern the heat rope under the cage. I find that keeping about 3-4" between runs works great. The drawback is that it takes alot man-hours to snake it around the underside of the cage and tape it down (I use aluminum foil tape) and you have to prop each cage up with spacers. I use pieces of 1/2" thick foam insulation for spacers.
Of course, you'll need a controller. I have a few of the Big Apple Herp Proportionals. I love those things. If you want one controller for all cages, get the heaters with the same watt-density - i.e. I use 23ft/50watt heat ropes for all cages. This way all cages see roughly the same heat output from the cables. This makes it easier to then use one controller if you want for multiple cages.
For my Colombians, I find that I need to use a heat lamp during the cooler months in addition to the belly heat mentioned above.
Flexwatt can be used if heat rope doesn't sound like your cup of tea. I have piles of Flexwatt now that I replaced it all with heat rope. You come pick it up, you can have it all 
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Mark G