pre-mixed is easier, in my opinion
I've used pre-mixed textured paint and I've used the texture that you mix in to your own paint. In my opinion, the pre-mixed is better. The one warning I will give you is that it doesn't go nearly as far as a traditional gallon of paint. The heavier the texture, the less coverage. I would suggest asking someone in the store how much you'll need for X square feet and then buy 1 more, just to be on the safe side.