Brush or Roller?

Two interior house painting (plaster) questions:

1) Whenever I paint with a brush, the longer I paint, the more the brush seems to drag. The videos one watches regarding painting, never mention this problem. I'm sure it happens to more painters out there than myself. Even if I reduce the paint with water/thinner, it just keeps on dragging. Is there a trick that is not mentioned in the media, like rinses the brush off in water/thinner every ten minutes or so? Thank you.

2) The ceilings are ten feet high. I told the paint store sales person I was planning on using a roller. He sold me some 4" and 6" brushes instead. Well, he's not the one going to have to climb up and down my rickety aluminum ladder a zillion times (after 3 coats). Roller or brush? Thank you.