Imagine the keyphrase you want to rank for "this is the keyphrase".

Now imagine you bought a domain name "http://thisis.net".

You still want to rank for the keyphrase "this is the keyphrase"

so which one would be better SEO wise (hover on link to see the difference):

1) thisis.net
OR
2) thisis.net
OR
3) thisis.net

Notice that in the original phrase there are spaces between the words. Are there any studies on - and _ symbol usage and ranking?