if the glass is 1/2 in thickness, you will notice a difference, if you have white gravel. but only if you look for it and run 10k lighting.
where it really is noticeable is when you have the tank against a white wall and you look through both front and back panes of glass.
low iron is "better", how much anyone will ever notice is based on how well trained there eye is.

this is just based on my personal experience and opinion. i work with glass daily.