Which is a better domain?

Simply looking at this from a cost perspective, without diving into arguments about the impact of the hyphen in SEO, the $270 for a hyphenated .com domain that includes both keywords is the most attractive choice.

SEO thread - hyphens are okay - in some cases better.

http://www.seomoz.org/q/hyphens-vs-underscores

keyword1-keyword2.com will return searches for both keywords.

Matt Cutts (of google fame) made a video in 2011 on underscores vs dashes - provides good info on how dashes are interpreted.

For a new site, there is zero percent possibility to rank a 500k keywords if this is not a decent site people going to like much.

If you want a site people likes much, domain name plays an important though not dominant role.

It looks cheap to stuff keywords on domain name. So WordAWordB.com is bad. WordA-WordB.com is worse. Gone are days of exact match domains.

If your domain looks nice and imply it's about WordA and WordB, that's much better. People will know it, Google will also know it after people.

What nice means is: dot com, no hyphen, unique, not too long, easy to read.

Search engines are getting smarter and smarter day by day. If you are building this website for long term engagement, you might wanna consider building your own identity without basing it on search results. What you need to build is your brand and authorship.

Time will come, this domain hype will be devalued. I myself get surprised sometimes when I search, domains of those helpful articles on search results are names of those authors or even just gibberish nonsense.

Lastly, you cannot get that domain on first page easily, especially with google penguin algorithm. If, however, you manage to do so in the near future, new powerful algorithm will be out and will ruin everything.

Instead of wasting your money on this premium domains, why not spend it on your development and marketing strategy. I suggest www.oneblueearth.com, same as your username. Good luck!