[ODE] What's better: create several spaces or decide which geom to collide in NearCallback?

[michael kapelko]

Since it's not possible to add the same geom to other spaces, I did all 
work in NearCallback to prevent unnecessary collisions. So I want to ask 
what's better: create necessary geoms in necessary spaces every step or 
prevent unnecessary collisions in NearCallback every step? The former 
seems to be easier (didn't try, though), the latter is a more 
complicated. What's more expensive in terms of performance? And still, 
what's better (way)?
Thanks.