Surely glass would be better than plastic because it can be recycled after it's life is over?
ok, I suppose that's technically correct, but you have to balance that against this:
Acrylic is stronger than glass, so that life will be a lot longer
acrylic is lighter than glass, so less fuel will be used transporting it
Acrylic insulates better than glass, so no matter what temp the aquarium is (tropical, sub-tropical, whatever) it will be easier to maintain a steady temp
Although people think it scratches easier (which it does) those scratches can be polished out, unlike glass, extending the useful life even further.
but I would also argue that the tank which would have least impact would be the tank which was going to be thrown away by someone else, until you took it out of the trash. you're stopping it being trashed (yes, even recycling uses energy) and it also means another unit wasn't produced for you. can't beat those savings from the manufacturing process.
I'd also say that 10 Gallon tanks would probably be best, since when you add more, you have more water to heat (if going tropical) and if going lower you aren't getting the energy saving of using smaller filters/heaters, since you'll still end up with something capable of maintaining a 10G so between 5 and 10 gallons would be your best bet.
and one last point, I would say plant the tank... they may be relatively small and underwater, but every little helps.