SEO : Are Domains with Dashes Better?

For search engine promotion purposes, yes.

1. Search engines consider a dash (-) to be a space

To a search engine, all of the following domain names pretty much read the same...

www.dietpills.com
www.diet-pills.com
www.diet--pills.com (yes you can have more than one '-' !)
www.diet---pills.com

2. ....And make more sense to the brain in search results

Normally the eye reads the words, sends the information to the brain, and the brain interprets that information into meaning.

However when presented with a large amount of information, the eye scans and the brain absorbs, in order to identify matches in a split second.

In effect your brain performs like a search engine, quickly evaluating to see if what it is being presented with matches what you want.

With 6 billion searches being performed every month, most of don't read - we search, scan and THEN then read just the entries that look right (ie. that our subconscious made sense of).

Follow so far? Look at the example below, showing 2 nearly identical search results. Literally blink at the results, which is how we often look at search results - which one made more sense?

In the context of a results page containing 1000 words, glancing at totally-safe-diet-pills.com made a lot more sense that seeing totallysafedietpills.com, where the eye had to check what the words were, and where they stopped & started.

Also the subconscious brain may have interpreted that it is seeing the word 'safety' spelt wrong as 'safedie' !