I personally think heat tape is better. I never experianced hot spots with it. And one single strip accross the back of your shelf is all you need. With the heat cable, I would imagine you will need to go back and forth a few times in order to cover enough surface area to actually be usefull.
If you are worried about hot spots, you could always do back heat. Run the 11 or 12" wide heat tape down the back of the rack, as opposed to laying a thin strip on each shelf. I beleive John Hollister uses Back Heat on his home built racks with no problems (this info from a tutorial he was working a couple years ago on on how to build simple rack systems).
That said, if you dont mind running the heat cable back and forth a couple times, and if you can make it so it doesnt get knocked out of place when you slide the tubs in and out, then it should be fine also.
Either one, be sure to use a thermostat to control temperature. You really dont want heat tape or heat cable left at 100% power all the time or your temps will go high.
-----
Billy
Reptile World
http://www.reptile-world.net