If its flat cable, it isn't cat 5 or cat6 - cat 5/6 specifications also refer to the form factor. Those cables may have the ame electrical specification as cat5 and 6, but that is as far as it goes.
There is a brief comparison between the electrical specification for the various categories of cable here:
http://www.xmultiple.com/xwebsite-forum24.htm
But unless you are running cable in excess of 100metres or more, using cat6 over cat 5e is a waste of money and a load of hassle because the cable cores are bigger, stiffer and the cable a lot harder to handle and terminate. If you really want to future-proof and you have deep pockets, lay fibre. Otherwise stick to cat 5e.