I run a straight cut box in one of my other machines and it is a seriously noisy bitch of a thing.
Reason I use one is that more ratios are available and also was having some side thrust load issues eating bearings and thrust washers.
Strength issue is a it of a red herring - it depends on the gear material - like for like material wise, a helical gear (the gear and teeth themselves) will be stronger as the load is spread over a larger tooth area - effectively more teeth are meshed. However the box as a package may well work out stronger as life is easier on thrust bearings in a straight cut box (as CW said).