I think you want a synthetic bristle brush. The naturals will pick up contamination over time and may cause some cross pollination that might cause some problems. If you have used the Hake brushes, they eventually get completely stained with sensitizer, even if you are agressive about cleaning them. You never know what that is doing to the print, but if you are consistant, then it doesn't really matter.

Simmons makes another brush that people have discussed, Simmons Skywash, I think.