3x is the only way to go for disc brakes, and you would be surprised how much you can feel the difference between butted and pg spokes! As said already, double butted spokes have inherent elasticity which makes them more durable, more comfortable and lighter. Also, brass nipples though heavier are much nicer to work with than alloy. I do however have a curve-ball with regards to nipples… DT Swiss Hex-lock nipples are spectacular. Ever since I stumbled upon them when rebuilding a prebuilt pair of DT wheels I have used them on all of my own wheels with no regrets. They are alloy with built in thread lock, so they are light, they stay put, and the thread lock actually makes them bind less whilst building. The especially nice thing about them however, is that they have a hex socket type head on the end, so you can build them with a driver tool from the inside of the rim getting them extremely tight with no chance whatsoever of chewing or rounding any of the wrench flats, yet they can still be adjusted if necessary with a traditional tool with the tyre and everything on at the trail side 🙂 They are quite expensive though…