i know this is a very old question. Where do you feel is the best hotel to stay at. I like being comfortable without being in a out dated room. I have narrowed it down to 3 properties. Bellagio Palazzo or the Cosmo. I booked a suite at each just to be safe until I decide. I like to hang out at hotel bars,  people watch,  during the day i go to the pools. I have heard that Cosmo has had some room security issues.  My favorite is the Wynn and Encore those have been ruled out due to a large convention at the property on my dates of travel.

I love the Cosmo. I like their rooms and love their rooftop pool. The suites are beautiful. Plus it’s always a plus to have a nice jacuzzi tub with a view.

You get a great room with a balcony perhaps a view of the fountains.
And no key card or security if you want to have some company.
I do not know about the security room issues you speak of.
2nd choice I would go with the Palazzo room over the Bellagio(unless you booked a real high end suite here)
Have fun!

Cirque Restaurant  
Conservatory
And Bellagio Fountains  
My Favorite Hotel

Not knowing your age, the Cosmo draws a younger crowd. The Bellagio has a more mature crowd but not like the old "Get off my lawn" grumpy group that is at Caesar....it's old money. But I like the Venus pool there so....
You mentioned the pool. The Ballagio has the best pool layout. Multiple pools with great sun exposure. Only thing missing is that they don't have a European theme pool like the Sunset Pool at the Wynn. But that's why they have strip clubs.
The Casino layout makes sense...easy to maneuver
People watch, the main path to the room goes right through the gaming area. Pull up a chair at the  blackjack table or hang out at the crap table and watch the crowd pass as they go to their room or the clubs if it's at night.
The Bacarat lounge and the side bars are also on the main path so you'll get an eyeful.
You have a suite so that should be ok but the standard rooms at the Bellagio are in need an upgrade. They haven't changed in 10 years.
Last...Location, location, location.....you're next to the Forum, Crystal, and the Miracle Mile shops. You'll be in the middle of most everything
Best Customer service...at least to me...."Welcome back Mr. Happy", "How was your flight Mr. Happy"...."Can I kiss your ass Mr. Happy"...lol.....Kidding on that last one..lol

Wow, what a great response! Seriously you sound better than most tour guides and it shows that you really know your way around town!

Was just out there a few weeks ago and had a wrap around terrace suite at the Cosmo (which are very nice).  I go back and forth between Cosmo and Bellagio depending on the specifics of the trip. Bellagio is a little more laid back vs Cosmo has a little more energy to it. Both great experiences just very different vibes.

I love this hotel ( former Mandarin Oriental ) and it is a walking distance from Aria and Cosmo to gamble or have a drink ( although the lounge on the 23nd floor is great! )