The glass and steel (18mm female) are pretty different so I'd think about how you'd use it. The taper in the glass joint allows you to use paper tubes, glass adapters, and RBT-style stems, and it holds heat well. The steel heats up quickly and works really nicely with wooden bowls and stems. Its joint is deeper so you can use RBT-style glass stems with a longer taper. Both work equally well with Alan's stems. My glass one has survived a drop from the desktop, and the steel should be indestructible.
The male heaters will fit like an injector so presumably most injector bowls should fit.