What's the difference between an 18-gauge brad nailer and a 15/16-gauge finish nailer?

18-gauge brad nailers shoot thinner brads (typically 5/8" to 2-1/8") that leave smaller holes, making them ideal for delicate trim, casing, and small moldings. 15- and 16-gauge finish nailers drive thicker, longer nails (up to about 2-1/2") with more holding power for baseboards, door casings, and heavier trim. Most reviewers suggest brad nailers for fine work and finish nailers when holding strength matters more than hole size.