Depends on where you want your traffic to come from, without an 's' is generally more memorable and with an 's' is generally more search engine friendly. There are exceptions to this of course, but the best thing to do is register both.
The reasoning behind the with an 's' option being more search engine friendly is that it contains both the plural and non plural version. Take namepros for instance, a search for both namepro and namepros would bring up the result namepros, hence you pick up both searches.