What paintball gun barrel is better for long distance long or short?

It depends on what you want.

If you're looking for low-priced accuracy, go for a 14 - 16 inch barrel. Contrary to popular belief, any barrel longer than that only makes the ball less accurate. It begins to bounce around the barrel, which clearly only makes it worse. ($30-$50)

If you're asking about general distance, either go with a Flatline or an Apex. Flatlines will give the ball backspin, which allows it to float a little. Apex barrels however, allow for you to choose the direction of the spin goes. These are two piece barrels, so having an 18 inch barrel in this case might be un-wieldly in close quarters, but it won't affect your accuracy when sniping. ($50-$80)