Two other things to consider with tempered vs annealed glass:
-Tempered glass is less optically clear than regular, not low-iron, annealed glass (the process they use to temper the glass causes some distortion which can make the glass appear much greener than regular annealed glass - the thicker the glass is, the more the visible the distortion is). Unless you're going with tempered glass specifically to use a thinner glass on a tank, the difference in optical clarity is likely to be pretty noticeable.
-From my understanding, the way tempered glass breaks can make it a bit of a safety hazard for larger tanks (I've heard for basically anything above a nano). When tempered glass cracks while under pressure (like when it's holding hundreds of pounds of waters), even if it's a relatively small crack, it basically loses it's structural integrity and shatters into a million tiny pieces. Normally, this is a good safety feature (as it's not dropping huge, killer glass shards when it breaks), but it's not so much a great feature for your fish or for the huge amount of water that would be pushing the tiny glass pieces out and away from the tank. With annealed glass (including low-iron glass), when it cracks under pressure, unless it's a pretty substantial crack, it retains a good amount of its structural integrity, so the glass has a much higher chance of holding firm until you can drain the tank. There'll obviously still be flooding in either scenario, but (in theory at least) it shouldn't be like panel bursting flooding with tiny shards of glass everywhere with the annealed.
Here are some articles discussing the features mentioned above:
With regards to low-iron vs standard annealed:
-Low-iron is obviously better optically (some people say they wouldn't notice unless they had a low-iron tank and a standard tank side by side, but others feel it makes a world of difference) and more expensive.
-Standard annealed is cheaper but optically inferior.
-Some people say that low-iron scratches more easily (this might be a concern for you, it might not be).
-Some people believe that at least some kinds of low-iron glass may degrade optically in time (like how acrylic can yellow overtime with UV exposure). To my knowledge, there is no scientific basis for this belief, but it is a belief some people have.
Personally, I would go with non-tempered, low-iron glass, but a lot this is preference/belief based for people, so you really just have to choose what glass you think would be best for your tastes.
