I have only owned a spring scale. I don't know if it was all that accurate but it sure was tuff. I would prefer a digital scale but the only problem I see with that is you would need to make sure it had fresh batteries
I prefer my spring scale Boga Grip. When I "bagged" a 22 lb snapping turtle while I was out flipping a frog in some weeds last fall, the BG took that snappper out of the water and I was actually able to retrieve and save a $7 SPRO frog. That alone is enough for me to recommend a good grip-type spring scale. Furthermore, I just find the gripper to be so darned useful when I happen to catch a catfish while I am out there.
I'd say new spring scales would be fine, but they don't stay new enough. In one of my last tournaments I was half way through one day culling. It seemed all my bass weighed the same nearly, making it tough to decide. After about 3 culls one apparent 2.5# bass of all 2-2.5# bass caught suddenly clicked down to 3#. It happened again, especially when jerking the scale up a little. I guess it had a rough spot from rust or some impact damage.
When doing fisheries work we had both, a spring scale and a very expensive digital scale. The accuracy of both matched, but like Keith wrote, the spring scale can't give you ounce increments, not being a precise instrument, though probably very accurate if you could just read ounces. Like also said, those ounces sure can make the difference. The digital scale was of course a lab quality model costing a couple thousand bucks. Very accurate and precise to 1/10,000 oz. Someone up the ladder decided we didn't need such precision, so the expensive one was replaced with a Rapala digital scale. It was good to 1/10 ounce and a lot more convenient. We tested it against the lab model and found the accuracy right on.
How many tournament fish have you seen weighed on a spring scale? Probably never in the last ten years. There is no guessing, no interpolating between spring scale divisions. The accuracy of digital is simply unbeatable. I carry two different brand digitals, Berkley and Rapala, both always agreeing, one at the bow, the other at the console. The bow scale hangs by the hook from a Go-To bait holder under a compartment lid. The other hangs from an access hole inside the console.
After all this time, I stand unrepentant in my faith of the utility of the Boga Grip. I hooked up a nice 5 lb Northern not long ago that the BG handled just fine. Whether it was 5 lb, or 5lb 2 oz, who cares? This thread has convinced me however, to purchase a Rapala digital for the ounce readings when it comes to bass. I'm happy with both for different reasons.