Obviously LLL.extensions are much more valuable IMO You can find some recent 3 Character domain prices at http://www.3character.com/recentsales.html
The L-L.extention formula looks bad to me. I'd rather allow it an extra 2-3 letters to get a decent brandable name, yes they are not all taken yet.
both are the same in the search bot's eyes but as for marketability, the domain.com is stronger than domain-name.com as domain-name.com is easily associated with spammy domains.
i would say if somebody wants to Speculate into domain trading, then 3 letter LLL.COM would be better.