my 240 is (96 x 24 x 24) is made of 1/2 in glass.
i do not have any wave motion or anything like that, so i am happy with 1/2
if your going with 4 vortechs and you think you will get a lot of back and forth motion and wave action, i would do the 3/4 mainly because you have more silicone holding the seams and there will be less stress.
if your vortechs are going to be on a simple flow pattern and just a softie tank, then 1/2 should be fine.
your almost doubling the price of the tank with 3/4 but your getting a lot more tank (weight) and security. the sad part is no one wants to sink more money up front, but later on they always wish they would have.........
i have never found anyone that wished they would have built a cheaper tank or thinner..
just my opinion.
i have front and back bracing with 4 cross braces. i like the multiple thinner bracing verses the 2 super wide ones.
but its your tank, and dont forget to start a build thread and keep us informed.
also, with a deeper tank, (front to back) i would look into bottom bracing. it is just added security and thats also where all the pressure is.