Depends on what Models, Hi-Po Quads can out run Trials Bikes, and small displacement Dirt Bikes, however late model 125's and up, Stock the Quads
Don't have a chance, unless their on Pea-gravel, really slick surfacing(Mud
cover'd hardpack, Ice, ECT)or the Bike rider can't drive.
definatly depends on make and modle. my cousin has a 3 wheeler that he takes to glamis every now and then it runs about 85 mph on a flat. and will pull a hill like you coulnt believe. but then again it is made from a v-magna motor cycle. but hey it still will outrun any dirtbike ive seen in my parts strickly becouse of the speed in the sand.
in my area a m/c cant keep the speed up in the sand and sinks real easy
and slows down. therefore i now ride a quad. i like the ease of use in the
sand. in this regard a quad will most likely run faster in the soft stuff. and
in mud i never could keep a m/c going (im talking soopy slosh.). if you ride in
hard pack or trails then the bike will definatly outrun on top end .
I still am inclined to believe a M/X'r or Enduro is faster in the Sand.
Stock that is, I do not think any Quad(Stock)could outrun a YZ400/426, a KX500
or a CR500 even in deep sand, mud is a different story, Quads there for sure,
MotoCross tracks have sand on them somtimes deep, and the 125's and 250's
are turning much faster times than Quads on similiar terrain.
However your points are valid, and worth pondering.