On my previous boat we installed Spurs, my current boat has the Shaft Shark or some equivalent...round blade without the scissors action of Spurs.
Jon's right, when we added the Spurs to the previous boat, we actually had to replace the shaft with a longer one so that there was enough space between the strut and the prop to fit it. I think the Shaft Shark takes up much less room and is likely to fit on an existing shaft.
With the old boat before adding Spurs in just one summer in Maine I hired diver's 3 times to remove birds nests of melted poly from lobsta pots. I asked one of the divers, who was a lobsta man, what he used on his lobsta boat. He said spurs. That took away the guilt factor of cutting cutting someone's gear. In some harbors the spacing between pots is less than your beam, so see and avoid doesn't always work.
After putting them on I never snagged a pot again. Now maybe I got lucky
I've had the Shaft Shark for 5 years. My current boat has a skeg hung rudder so the prop is in an aperture, which provides some protection. Last year I managed to tangle a pot, and it didn't cut loose. Luckily we had a hooknife like in Jon's post. From the dingy, hanging over the side, whilst drifting I was able to untangle/cut the birds nest off without getting wet beyond the elbow. Luckily it was relatively calm with light winds.
So I don't know...my guess is the scissors action is better than a round knife, but in either case be ready to swim and/or have a hooknife and a good pole that extends far enough to reach your prop.
I'm always happy when we hit the Canadian border where lobsta fishing is in the winter
