Height is the most important factor when figuring out the thickness - for a tank that size, 3/8" is the thinnest I'd go; 1/2" would be extra secure.
From everything I've heard, overbuilding a tank is a better idea than underbuilding one (especially if you're not an experienced tank builder), so I'd personally suggest going with the 1/2" glass and fully bracing the aquarium (eurobracing and crossbracing) to ensure it's built to last.