I have limited personal experience with prop guards, however someone I know once fitted one to their engine, as they were using it as a sailing support vessel and their club required it at the time (quite a while ago). Since then, the RYA changed their guidance so he removed it. When the time came to upgrade his engine, he found that the resulting holes brought down his resale value quite a bit.
From what I understand, the plastic prop guards wonít protect the prop as much as they will protect something in the water. (I.e. theyíll help protect a swimmer in the water but they wonít do much if you hit a large rock in shallow water)
It is therefore my opinion that if your main aim is to protect your new engine, donít fit one and just be extra careful in shallow water. However, if you are often operating with swimmers then it may be worth it.
At the end of the day itís your decision, however just be sure of your choice, as thereís no going back once youíve drilled the holes. Make sure you get them in the right place first time otherwise youíre looking at replacing the lower leg.