Hardtail, it is much easier to sprint up hill on a HT. If you do true freeride then a FS, but most people buy a FR rig and ride only XC. As far as technical descents, there is nothing better than passing a bling bling full suspension bike on a stiff aluminum hardtail. Most people who recommend full suspension for XC are the same ones who look at what name is on your bike first thing. I have owned both and after one season traded my FS in on a HT.