Use Purdy rollers. After loading roller, put paint on the wall, not overlapping the first pass. Once the majority of the paint is off the roller and on the wall, including ridges from extra on the edges of the roller, use the roller to spread it all out thin and smooth. The most important part is the spreading it out without trying to use a loaded roller. It only takes a thin coat to cover.
Paint brush, quality, with real bristles, every time.
You'll use less paint, clean-up is easier, and it's often quicker, as you don't need to tape everything beforehand....and get it stuck under your finger-nails trying to remove it afterwards.
Plus, the finish is nicer!
Just make sure you have adequate ventilation and humidity - if the room is too dry, the paint will dry too fast and you're liable to get streaks....this applies to paint applied by brush and roller both.
What nap do you use for plaster walls in an old house? I tried a roller once and practically went crazy trying to get all of the nooks and crannies filled in. After that, it was back to the brush for me.
I use the paint pads and the thin rollers. If I have a large open area to paint the thin paint rollers work well. In smaller areas the paint pads work better, particularly near the ceiling, around windows. I have more control with them.