Opti clear is a brand name for Low Ion glass, there are several brands in the market
I question wether you will see any real difference in low ion over float for a standard small tank using 5 or 6mm glass
if the background was with or similar, yes it could be an advantage
I strongly recomend that you view both by looking at a similar coloured background before paying good money after bad.
Strengthened, heat strengthened, toughened and tempered add 5 times the strenght to the glass. That is 5mm glass that has been toughened will have the strenght of 25mm glass. Diver is corect when he advises not to use toughened/strenghtened as once scratched its already possibly going to break. Basicly, toughened glass is stronger because the outside surfaces have been shrunk creating tension on the inside of the glass. This tension (or energy) creates strenght. Once scratched (enought so that you can feel it with your finger nail), the surface is damages and can frelease the internal tension/presure from that fault. It may never "release" but it can at any stage. Keep to annealed glass.
The National Aquarium in Napier has standard annealed (not toughened) glass, 35mm thick.