Hi Cruisin' Lori,
I don't know anything about the glass bottom boat tour. But, the Stingray City, I have taken last October. FABULOUS! I booked it through Carnival. No matter who you book it through, they all take you to the same place to swim with the Stingray's. So, the crowded conditions are the same in the water. The only thing different is booking privately might not have so many people on the boat. I did not think it was sooooooo crowded. If you could do both by booking with Carnival, I say Go-Ahead. Some excursions have multiple trips in one day. Maybe you can take an early trip with the Stingrays and later trip in the Glass bottom boat. Or vise versa. Your travel planner should be able to tell you how many trips for both of those excursion they have in each day. I will look over my papers for excursions when I get home tonight and get back with you tomorrow.