What's special about ahi tuna?
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.