There are always folding marlinspike knives. Some are quite well made and the spike could come real handy if you snug a knot too tight.
Since I have a plethora of knives I have made and I tie mainly using synthetic materials, I personally would opt for a heat cutter. Then again, I don't cut all that much rope, and cords are easy enough to melt the ends of (or whip).
The main thing is to cut on a surface that will not damage the edge of whatever you end up using to cut with.