Two domains is better but you have to optimise (esp off-page) two sites. Provided you avoid excessive inter-linking, you may get up to four results in some searches (100 per page).

I would say if you have the resources to build and promote two sites without diluting your marketing at all, then two domains with two separate sites would work. Unfortunately most of us don't have the resources to promote two sites at the same time for the same niche. For the latter, I would recommend one stronger site. Either way you should pickup both name since they are cheap and 301 one to the other if you decide to build only one site. The 301 will provide little value other than locking down the name from someone else to take, unless the name already has inbound links to it, in which case you might get a little juice from it.