I've had spring scales since the 1970's. They are pretty good. I have a brand new spring scale (cost me at least $50).
I bought a digital scale from Easton Technical Products this past spring (they are expensive at $150 though). I LOVE these digital scales! I've checked their accuracy with free weights and they are right on the money. I use mine all the time to check specs and measure my draw weight at my draw length. This is one of my favorite archery tuning devices. There may be other digital scales out there that are more economical. However, the Easton scales are used in FITA tournmanents to check the legality of bows so they must be noted for accuracy?