Pure theoretically speaking longer barrels are more accurate. after all, the tip of your barrel is a few inches closer to your target.
Also in theory, they should be more economic, since the gas cloud that propels the paintball down the barrel has more room to expand while accelerating the paintball down the barrel, meaning you need less gas to get the same velocity. However, this is mostly negated by the fact that most barrels are ported and the none ported part of the barrel is often a standard length. You could even argue that a longer barrel uses more air if ported, since the gas cloud escapes in the ported bit and so stops accelerating the paintball, meaning it encounters friction without driving force over a longer distance in the barrel.
In reality accuracy is mostly decided by two factors: The consistency of the valve (as in does it release the exact same amount of air everytime you pull the trigger) and the consistency of your paintballs (as in are the paintballs exactly the same size in the batch you're shooting and are they all the same exact shape).