Hyphen or not hyphen?

Well, for SEO, they're basically the same. Search engines are smart enough to see the keywords.

If I were you, I would go for the domain name without the hyphens since it's also a lot easier to type in.

However, I think that having a short, catchy domain name is a lot better. And if it has some SEO benefits, then that's good.

(Note: This is if you're building a website you're going to be in for a long time... Niche sites could just be keywords).

Matt Cutts talks about hypens for urls, but domains?
That's another matter.
Domains can rank for whatever they want.

Highdefinition said it best:
You want to separate words in a domain so as not to run
words together that can be misconstrued.

An underscore should never be an option. An underscore looks
like a blank. Dashes are more likely (99.99%) to be seen as
a space.

If people are relying on keywords in a domain to give them some
sort of boost, then logically speaking, it should make no difference,
if we are going by sheer words. student-loan and student_loan
have the same words, right?

But google does make a distinction between one_two and one-two.
one_two may appear to be one word. (plug in whatever you want
for one and two). If you use the underscore, people will need
to be searching for one_two in order for your site to most likely
come up. But who searches for that? People search for one two.
And that's why the dash is important. Now one-two will show up.

Let's quote an old Matt Cutts blog post:
"So if you have a url like word1_word2, Google will only return that page if the user searches for word1_word2 (which almost never happens). If you have a url like word1-word2, that page can be returned for the searches word1, word2, and even “word1 word2″ "

If you don't put a dash, and your words can meld together, then
google may have a hard time figuring out exactly what words are
there if it is not a common phrase.

Google does not penalize for underscores, but may not treat it the
way it was intended.

Paul