The size rating means the largest particle the filter will allow to pass - so a smaller number is more filtration, right? I could have sworn when I bought that paper style filter the rating was something huge like 250 microns or something.
Yes, smaller number, finer screen.
That'd be pretty odd, at that point I think they'd have to go to a plastic screen in there, as the paper fibers would need to be spaced too far apart to have any strength.
Eh, either way, important part is that you've got some sort of filter on there. You've got a carb, so it doesn't care so long as the big shit's screened out. Using it off road'll keep the fuel sloshing around enough that anything that accumulates in the float bowls will be stirred up and sent on its way. Just run the aluminum filter, put it before your pump. Less failure points.