I don't think there is any 'best', it's more what you like.
I use flexwatt because that is what is convenient and available for me. I do also have an Animal Plastics T-10 cage that has a heat cable and I have no trouble with it.
It really depends on what you have and what you want to do IMO.
If you have racks that aren't custom built with recesses for the heat cable, then I would us flexwatt. But if you have a rack that has the recesses for cable then use cable. Both work great, but cable is a pain in the butt if your rack wasn't initially designed for it. Or at least that's been my experience thus far.
Matt
http://www.elusivereptiles.com
Specializing in ball python morphs
1.0 Spider - Rising Sun
2.0 Pastel - Old Yeller, Gnarles
2.0 Normal - Sir Hiss, Gargamel
0.1 Normal 100% Het Albino - Chuckles
0.1 Albino - Starfire
0.1 Reduced Pattern / Banded (tiger) - Bertha
0.2 Normal - Thulsa doom, Azriel
0.1 Normal (blackback) - Evalynn
0.1 Oddball - Georgia