I`m not really too sure which bearing would be best to use under very dusty conditions....the conditions we race in here tend to not have a great deal of dust, yet somehow the bearings still seem to attract it like a magnet.
Acer Racing seem to have a good variety of bearings, so it may be worthwhile to send them an email and ask their opinion of what bearing would work well in your conditions:-
One bearing to keep a close eye on, is the small clutch bearing (5 x 8 x 2.5).......this bearing is very small and operates in a very harsh environment, being out of sight it`s easy to neglect it.
I`ve had one of these bearings "die" on race day and the metal shards from it also trashed the thrust bearing.....keep it clean and well lubed and it should be okay.
I always keep a spare of this bearing, as well as a spare thrust bearing in my toolbox.