Really nothing to do with fashion. It's all about SLR. A 904 would work fine if you get 1st gear right. And if it has a real tight powerband and benefits from less RPM drop on the gear change, it would work better. Pro Trans sells gear sets with a much larger selection of ratios than is available for Glides, so you can get 1st gear exactly where you want it. Sort of like the difference between a 9" and a D60.

I like the 904. Unlike the 727, I think it's just as fast as a Glide when properly done. But durability, aftermarket cases and parts support, converter selection, cost until you get into a real high end unit, all favor the 2 speed.