In my case, I never register just one version if the other is available. I register both or do not register any.
There are several reasons for this. Since I acquire names to offer them to end-users, my offer is irrelevant if they can go register the one that's available.
This happened to me when I was a newbie and I learned from my mistake.
So, if I register CityPlumber.com and CityPlumbers.com is also available, it makes little sense to approach end-users with my version.
Again, I am only talking about singular and plural versions that make sense.