I've had 20 or more orders with IST in the last year, and I think 3 with CGN.
On shipping time they're about the same. I've had books from IST ship the day after I ordered, and I've had it take up to 2 weeks because a book was backordered. Usually it's a few days. CGN is usually within a few days too, but when I ordered 4 or 5 books during their Black Friday sale it took them 2 weeks to ship them.
On packaging quality, I give IST the edge, but they're both quite good. IST uses Insta-Pak foam pads that are specifically formed to fit your books and the box. CGN uses newspaper or foam peanuts.
The reasons I prefer IST over CGN are:
CGN's $7 flat rate shipping vs. IST's $4 flat rate or free shipping over $50. CGN will often have slightly better prices on the books, but not enough to make up for the $7 shipping.
This is a big one. IST has better customer service, hands down. They have always been great the few times I've received damaged books, and the time I got a book with major printing flaws. You do have to contact them about it within 48 hours though.
Take a look at CGN's returns policy though. It's pretty clear that you need to purchase the additional shipping insurance when you buy from them, or else they might not replace damaged books.
Also, in my interactions with their customer service through email, IST has always been courteous, and CGN has been rude at times.
Last thing, and I've told the long version of this story before, but... CGN took a book out of my order because they forgot to mark it as "Limit 1 per customer." I wanted to then add another book to offset the $7 shipping and they told me they'd have to charge me a 10% cancellation fee for that. When I said that didn't seem fair since it was their mistake, the guy pointed me to their TOS which does say they can do exactly that, change your order without your permission.