With the one piece mount you don't have to check if the rings are inline with each other even if they arn't. There is nothing you can do. With reaming or lapping that depends on the quality of the rings. If the machining is good on the rings you wont have too. Cant really give you any pros or cons between the two. If used them both the only time I buy the one piece is if i need more eye relief and have two push the scope forward.