Ask for glass thickness, hardware brand and finish, and whether the price includes measurement, delivery and installation, because the cheap quote often turns out to be glass only with the fitting extra. That alone usually explains half the spread. Then ask what the warranty covers and for how long, separately for glass and hardware. When I priced a New York shower door replacement, Glass Factory itemized it that way and the middle quote turned out to be better value than the cheapest once fitting was added. Get the hardware finish confirmed in writing as well, since substitutions there are common.