Search Engine bots reads a dash in the domain name as a space. Therefore, if your primary keyword is more than one word, you will probably get better rankings if you use the dash to seperate your keywords in your domain name.
I have tried both methods and there is definitely an improvement in rankings using dashes.
I agree to get both domain names and set up a permanent redirect. Set up your site on the domain name with dashes, but market the other one.
Just my 2c worth…