The digital trigger gauges are accurate to +or- a fraction of an ounce. The mechanical types to +or- a couple ounces.
I have the Lyman digital, older 9v version.
Its looks cheap in pictures, but its solid. Its surprisingly well built, feels heavy and solid it the hand and is as accurate as you could readonably need.
Ive heard the wheeler digital feels lighter and cheaper, but ive not handled one.
The Lyman is a good tool. You wont be dissapointed.