That was one of the large deciding factors, the protection the keel afforded the single screw.
I suspect it depends on where one boats as to the vulnerability. While there is no question a prop deeper in the water and behind a keel is somewhat more protected than a pair of props closer to the surface and behind a pair of V-struts, it's rather ironic that we know more boaters (power and sail) who have fouled props or damaged props in single engine boats than we know who have done this in twins. The single engine boats range from Tollycraft 26s to a 60 foot ketch.
My own opinion is that some of this--- maybe a lot of this--- is due to the fact, that in our experience and observation,
most drivers of multi-engine boats seem to be well aware of their running gear's vulnerability and so pay closer attention to what's in front of them than the drivers of single engine boats (again, power and sail).
We have had several single-engine boat owners say to us that they have nothing to worry about with regards to the crab pot floats, logs, branches, and other debris in the water here because their prop is protected behind their keel.
I don't deny the additional protection, but if the attitude is "damn the debris, full speed ahead" it can lead to problems. While just a single data point, it's significant (in my opinion) that one of the most vocal proponents of his single propeller's invulnerability is one of the aforementioned folks who subsequently wiped out a prop on a piece of debris in the water.
While we would never own a single engine cruising boat ourselves for several reasons, were we making a choice between a single and a multi-engine boat, prop vulnerability would not be part of the decision-making process. We have run a multi-engine boat for seventeen years in waters that have a deserved reputation for having a lot of "stuff" in them and have only had a close-encounter of the debris kind once, when a rudder snagged a submerged length of drifting line.
These 17 years, plus the experience of a lot of other multi-engine boat owners we know who have a lot more experience up here than we do, tells us that while a single may be less vulnerable by design, in reality, it's pretty much six of one, half dozen of the other when it comes to running gear damage.
This may not be the case in other boating regions.
We keep a sharp lookout for stuff in the water, and since we removed the autopilot from this particular boat when we bought it, someone is hand steering all the time. This tends to keep one's attention focused on where one is going. Not that one can't do exactly the same with an autopilot, but the autobox does make it easier to allow one's attention to wander away from the water in front of the boat.