The only difference is the heat range. The 7s are one grade cooler than the 6s. If you are using the 6 version and the insulators look white after use and you have trouble getting full boost, especially in hot weather, switch to the 7 range. If you are using 7s and the insulator is getting dark brown or building up deposits, go to the 6 heat.
Incidentally, it varies by manufacturer if increasing numbers are increasing or decreasing heat. Most of them use higher numbers for higher heat.