Glass is usually better, unless weight is an issue (remember water is heavy in large amounts). Both work fine, but plastic gets scratches more easily. Silicone will only attach to glass, so I'm not sure how to assemble plastic parts (assuming this is one of your projects ). When I needed to glue plastic, I use heated glue ("glue gun"), but I'm not sure it will be tight enough for an aquarium.
Well the pieces of glass I have are quite thick, they were cut for me from old glass shelves to use for reverse glass paintings, which I never got around to painting. They were cut in squares so it would end up as a cube shape. Only about a gallon size.
I better figure out a lot more before I go making something and filling it with water and having it spill all over
I guess I would also have to re enforce the outside angles with something as well.
Maybe I'd better stick to a smaller easier project at first
3 species unidentified slugs