it is not the matter of what balls you have, it is the shield/seal that make differences.
for off-road, you have so much power to overcome the minor drag if there is any. it is much more important that nothing get into bearing. therefore metal shield isn't a good idea at all. things get into metal seild much easier. for off-road application please use rubber seal to at least prevent more stuff getting in.
also due to the abusive nature of off-road racing bearing break often, especially not clean up after time after you gone home from a muddy race, therefore extra replacement bearing is very important, if you have very expansive bearing set, when they gone bad which cannot avoid, you will feel the burden.
metal seal is for on-road car for the race are less abusive and the tract is clean.
Xray X12 2015
ARC R10 ATS