The thinner the aluminum the cheaper - you just don't want something that will be super flimsy and unwieldy. The situation with aluminum is that it's highly reflective compared to other metals, insofar as the percentage of the spectrum that it reflects. It is only out-shined by silver, so maybe silver-plating an aluminum hologram would be a good way to really shine it up. In my experience just polishing up the face of a freshly cut hologram with a rag and some polishing compound can just about double the brightness of the hologram's groove optics and thus the hologram itself.
Aluminum is also pretty soft compared to other metals and lends itself well to having the optics scored into it just by dragging a hard implement over its surface like carbide/diamond. I'm sure other metals will work to some extent, but will be harder to cut and also not as brightly reflective as the grooves will not form as easily, depending on how hard the metal is, and may come out with a rough finish that doesn't reflect light as a nice little glint from one spot.
I have not yet used any plastics but I know that holograms can be made which instead of reflecting light off the face of the hologram they instead refract it from the backside of the hologram. Not sure Holocraft is yet up to the task without spending a day of adding some modifications to code - but it's worth experimenting and looking into.
Plastics may require some finesse, however. I'm sure that reflective specular holograms can be made from dark glossy plastics but I have no experience with making holograms from plastic that yields acceptable results. Maybe coating the plastic afterwards. There's not really an easy way to polish up the plastic without marring it's gloss - not like with aluminum where you can just buff it up with a rag and have all the groove optics become much shiner. Maybe fume polishing?
Also, there's another aluminum supplier that always has 15% off sales (or thereabouts) just sign up for their mailing list. Their prices are competitive with McMaster-Carr: www.onlinemetals.com - 0.025" 5052-H32 Aluminum Sheet I usually go with 1100 or 3003 alloys because they're more pure, and thus softer/shiner but 5052 is pretty cheap right now and is equally as soft and only slightly less pure. Having followed aluminum prices over the last few years: less than $3.50 for a 12"x12" sheet is a really good deal! I wouldn't pass it up if I were actively making holograms.
Thanks for the questions!