What's special about ahi tuna? When I see tuna steaks, seared rare, on restaurant menus it's almost always Ahi tuna specifically.  What's special about Ahi with this preparation method?  Does it have different cooking properties from other kinds of tuna steak, like Yellowfin?  Or is it an economic matter?

I ask partly out of curiosity and partly because I'd like to make this myself, so want to know if I should be preferring Ahi to other tunas that are sometimes at the fish market.
As Jolenealaska mentioned in his reply, Ahi is generally yellowfin tuna. 

I believe you probably see it on restaurant menus a lot either because it is part of your local fish stock or possibly as part of marketing tactics used by the restaurants.

All of the true tuna fish species will be nice grilled, with some minor difference in flavor. Personally I would choose the tuna based on sustainability and freshness rather than specie.