Mountain Bike or Dirt Bike?

Ok title says it all, I can get a new $2k mid-line mtb or a 2003ish rm,cr,kx etc. 125cc dirt bike. Im stuck between the 2. MTB might be used a bit more because I can ride it on the street and stuff like that. But the thing is im afraid that I might get bored with it where as dirt bikes are ALWAYS...!!!ALWAYS!!! fun lol. They both cost the same to maintain (mtb stuff has gotten fricken expensive!). I have an abundance of trails for both so places to ride is not a problem. Well any opinions..?