For up to 30 x 40 (Frame Size), I will generally use Single thickness, either convervation True Vue or just UV. It's very dependent on the buyer.
Average buyer for me, won't want to pay the extra for the True Vue Convervation Glass, however it makes a big difference especially on the large prints.
I have never used Plexi, mainly due to the static issue on large sheets and getting it totally clean. Plexi, which won't break, will scratch easily. At least the brands I have seen, and once scratched, it will need to be replaced.
On the larger prints, I do, you can easily wire the Frame to handle the weight of the glass. I move around these sized prints alot and have never had any glass break, but you do have to watch the frame as it can also scratch.