Unless you have very deep pockets, it is best to concentrate on long tail terms and lots of them. As they convert better anyhow.
For me, I am only concerned with my rankings in google.co.uk, if I rank in the others then that is a bonus.
I got for a myriad of long tail terms, and one or two big terms.
Having ranked as high as #3 for online casinos in google.co.uk and currently sitting at #6, i can tell you now, that the non plural term ie online casino gets a lot more traffic than then plural version.