either way the glass is going to be sealed in a frame wich is then used in the setup, water level will aprox level with top of glass.. Now i have the option of one solid 15mm or 5 x 6mm for the same price..
Multiple panes has the problem of fogging between the panes, it could look really bad and be really hard to fix if not done perfectly. Also 5x 6mm would be really thick, and have tons of room for error.
I don't think that 5 sheets of 6mm put together is going to be stronger than 1 15mm sheet of glass. Also 15mm isn't thick enough to be safe with a 50" tall tank. I used this calculator to calculate my own tank glass. http://www.theaquatools.com/building-your-aquarium For that glass size if you where going to have water at 50 inches you would need 28 mm glass or over an inch. If you still want to used 15 mm thick glass you could go up to 34 inches of window opening hight and the water level shouldn't exceed the top window height by much.