Based on their designation, they must have an STC rating.

Ask them for that. That is what will tell you how effective they are going to be.

How are your current windows constructed?

They gave me a quote on STC 40, 46 & 48, single hung windows.
My current windows dual pane casements,, low E and are only an STC of 28.
The Silent Guard seem like the answer but if I go with the STC 46, they have 3 panes but none of them are laminate glass- which is what I heard you need? The STC 48 has laminated glass but is more expensive.

You don't "need" laminated glass for STC performance, but as you can see it does drive it up a bit and certainly translates in price.

Laminated glass is usually the best thing you can do in a standard replacement line up as most people don't offer offset glazing or other STC principled options.

STC - 46 is huge. Your windows will be much more sound blocking than your walls at that point.

Going to 48 is great, but probably largely unnecessary.