. . . But OTOH a student starting intro to tech doesn't know what works for them or doesn't. Like right now as a prospective student for intro to tech I don't know if I want to go back mounted doubles, side mount, or a mixture of both depending on the situation. How do I know that unless I try both? And I personally can't afford to buy both setups in the short term, but an instructor that is able to provide some standard equipment like spare wings, regs, and tanks can allow a student to try both with minimal investment which is a value added to students.
I am not convinced that a prospective student who has no idea what kind of configuration appeals to him is going to have an epiphany and know what's right for him after a few test dives with different configurations. My thinking is to choose an instructor who does a lot of diving of the type you would like to do, and whom you think you would like to model your diving after. The instructor is likely going to have a configuration preference--maybe he's a sidemount guy, maybe he's a backmount guy. If he dives a lot, he's likely to have some gear of that type to offer you to use for the course, or at least know of a source from which to get it for you to use. Assuming the course goes well, dive and dive the way your instructor taught you, until you get good with that gear. Then, if and when you find yourself attracted to some other configuration, you'll know you're ready to try it.
This is an admittedly goofy analogy, but it's something I have had in my head for some time. Few kids said, "mom, I really want to learn to play the piano." No, we (at least back in my day) wanted to learn electric guitar. But your mom knew that if you learned to play some basic instrument, you could switch to something else later, once you really had a grasp of music. The most basic basics of technical diving, like precision buoyancy, trim, propulsion, and control, are actually quite similar across the configurations. Maybe like the ability to read music?