a problem with the helix cut gears is that the housings are not set up to deal with the thrust loading of the angle cut gears

on the straight cuty gears---there are different combos that will be quieter than others as the backlash clearance is different between various ratios------they use a certain tooth shape/size, and so many teeth will define a certain diameter gear--without being able to change the shaft centerline, you get noise on all except the perfect combo---whatever that is.