I've done both, and as everyone says, these are 2 of the top shelf kits available.
Rosso Fortissimo is a super Tuscan style blend, and easily one of the crowd favorites that I've served. Super versatile, drinkable young, awesome after a year.... it's probably the single best wine I know of for a "if I had to choose just one" red wine.
Red Mountain is an incredible Cab Sav, I do not know of it's equal aside from a few commercial vins. The only minus point I have for the Red Mountain is the body is slightly lighter than the $100/bottle California Cab Sav examples I've tasted. The bouquet is ENORMOUS for a Cab Sav. It is not an early drinker... and needs 1-5 years or even more. After a year+ you will know what you paid those extra dollars for.
My advice is simply to make both.
If you want a little more advice (that you aren't asking for) add the CC Amarone and Barbera to your cellar. CC has a Syrah that's awesome as well. Even though CC was sold, so far it doesn't seem to have hurt the kit quality (but the prices are going up).