There are a lot of opinions on this topic.
One thing... look at the packaging for the batting. It'll tell you if it's suitable for tying. If not, you may have to make the knots closer together (don't want bunching), but it still gets done quicker than quilting.
Other thing... as far as "thread". I've used embroidery floss. TONS of color options and reasonably cheap. This way, I can match the colors to the colors of the quilt and the knots don't stand out as much.