This depends. Are you climbing at an area with lots of pin scars? e.g. Yosemite? If so, offset micronuts (such as DMM peenuts, or DMM brass offsets) are ideal. If not, however, then I find that regular (non-offset) micronuts typically work better than offsets. The brass vs aluminum option is about stickiness vs durability. Brass nuts will deform to the crack and stick better in marginal placements, but as a consequence they are less durable and will eventually get all deformed especially if you actually weight them/fall on them. Aluminum nuts will last longer but they won't stick as well in marginal placements.