Well, I guess it depends on the type of street.. if it's going as big as you can and tricking off the tiniest things you can find, or pretty much anything involving tricks, a BMX would be best.
If it's just airing out of things, or dropping staircases and throwing in an x-up or tabletop here and there, I guess an street MTB would save your ass a little.
If it's urban trials, you're not gonna be happy with a street MTB (especially if it's an SS and geared properly for street)...
And clifford_bgrddg, watch a BMX video with street and park sometime. You'd be surprised how big they do go.