Just to bring these all together... Here's the stock Vans fork, photo from bowenaero.com. Note that the bottom of the pivot is below the axle of the wheel. For a 6" wheel, this is about 2.5" of clearance.
Here's the Vince / Screaming Eagle / SE fork. Note the increased clearance as the bottom of the pivot is now raised above the axle of the wheel (green line), and the remaining hardware above that can catch on obstructions (red line). It looks like about 3.5"-4" ground clearance for a 6" wheel.
Here's the Bell fork. Note that almost the entire wheel is exposed to the front (green line). The bottom of the pivot is only just slightly below the top of the tire. The only remaining thing that can catch now is the vertical part of the stock Vans knuckle on the stinger (red line). This would be about 5" clearance on a 6" wheel.
And finally, here's the API fork, also linked from bowenaero.com. Note even better clearance over the Bell fork... The bottom of the pivot appears to be just at or above the top of the wheel, for about 6" of clearance for a 6" wheel, and the pivot itself doesn't hang as far below the stinger. This does require match drilling the holes in your stinger to the new knuckle.
They all work on smooth pavement. But I think the photos speak for themselves... The Bell and API forks definitely have the edge on clearance. If you did get a tailwheel behind something, like a block of concrete, the first thing to hit as you tried to move forward would be the wheel, which would roll. On the others with lower pivots, if you hit something, it's metal hitting the object, which could bind and lock you in position.
Disclaimer: I have a Bell fork on my -6, but have no financial interest in any of these suppliers