Both styles spin the arrow better than just a typical straight fletch. An arrow spinning is more stable and is more preferrable for someone using exposed blade broadheads. The difference between the two really is the amount of twist in the fletchings. Think of a straight offset as almost being a helical with less twist. The helical fletching will provide more stability because it causes the arrow to spin faster however, it can also offer potential fletching clearance problems with some rests. The offset fletch still spins the arrow and provides better stability than a straight fletch but does not have as much of a fletching clearance issue in most cases.