Simple enough arteries have thick muscular walls surrounding them making them very tough and elastic.(Trust me I tried using a dull scalpel in my gross anatomy class on a human aorta... didn't work very well) There is enough play in an artery that a dull blade would just push it to the side instead of cutting it. (Veins however are thin walled and easier to tear). With regards to clotting times, as long as you have neat edges to any cut there really shouldn't be any differences in clotting times. The "increased clotting" you are talking about in torn cuts is not due to increased platelet effectiveness but due to the fact that the major arteries were not damaged!
Obviously I vote the sharper the better