The best scale is the one they stock in Wal Mart/Target, etc. They're essentially all the same except for how high they go...ie..some only weigh up to 3000 grams and some go higher...5000 grams etc.

As far as weighing prey...if you have nothing better to do with your time than have at it. It is, however, completely unecessary. Use the "eye-ball" method and a little common sense. Keeping balls is not rocket surgery...don't over-complicate it...