I've used both knots and loops. These days tend to loops because I'm more confident that the ones I sew will hold. A knot takes up more strap length to make a loop, something closer to a foot than a couple of inches.
Using high test straps of the type usually talked about here a good knot won't weaken the strap to anywhere near a danger point. My favorite knot in all the world is the bowline. On cord the knot reduces the strength by around 50% (see
this discussion on knots and rope strength if you're interested). A loop made with a bowline does not slip, and can be easily undone after being under tension.
The knot strength link above cites experts who explain that the weakening of the cord comes from sharp bends, which force the fibers on the outside of the bend to carry the load. Makes me wonder then about the strength advantage of a sewn loop, because there's a sharp bend at the top of the loop under tension, bending around a segment of webbing just as would happen inside of a knot. Hmm.
Grizz