I certainly agree that going with a glass shop will provide you with better quality materials than from a box store. That said, any thickness of glass will keep the water contained. 3/8" is the "standard offering" from all shops we priced. The thicker glass will have more weight to slide (roll) nicer. And if you plan to install towel bars the glass will be more substantial.

We recently replaced our shower enclosure. We went with 1/2" on recommendation of the glass shop because ours is semi-frameless (track against one wall and bottom on a "pony wall" because they weren't perfectly plumb/square), has a robe hook drilled through and a 30" door and he felt the less flex from heavier glass would be better. It is substantial and looks so too, we're pleased and it was a $200 upcharge over 3/8".

I suspect the "Free Diamond Fusion Glass Treatment" is to minimize water spotting but you'll still need to squeege following use. We opted not to have ours treated as we are accustomed to drying the glass and understand the treatment wears away with use.

I have bypass sliding doors on a shower and I went with the 3/8" glass. I'm happy I did. The door is very solid and does not rattle at all.

My contractor told me that 1/4" would work fine (my shower is only 48" wide), but the heavier glass would feel more substantial. I decided since I was already spending a boatload for the bathroom to spend the extra $200 and get the heavier glass. My door was a little over $800, so your price quote seems comparable.

3/8" I have both 3/8" and 1/4"and the thicker glass is much better, slides easier, no rattles.

3/8" or better yet, 1/2".
Starphire.

Slider 3/8" is more than sufficient, 1/2" is unnecessary.

A framed 3/8" thick door is unusual - usually you do not need it to be this thick because the frame provides additional structure/stability. 1/4" should be sufficient with a frame around it.