They both are great ways to get you up and running but they are not competing boilerplates. In fact, they work quite well together. I would like SASS to be brought into Twitter Bootstrap instead of LESS but I’m sure that will soon be on its way. But for now the easy way to combine these two great boilerplates is to use http://www.initializr.com/.
It is fairly straightforward and once you arrive on the site you can click what you need. Pretty cool stuff. Big time saver. This article talks about the two boilerplates in more depth: