AT&T has 3 Apple phones, so you're wrong.
3GS, 4, 4S.
All of them have different iOS builds and different hardware in them.
They also have 3G iPad and iPad 2s.
So that's 5 Apple iOS devices AT&T carriest.
iOS updates come straight from Apple. The carrier does do testing but they do not do the type of backburnering that they do with other platforms (Symbian, WinMo, WP7, Android, even WebOS and BB) with Apple devices.
When the Apple update comes out for the iPhone on AT&T it's there for Verizon and Sprint as well as there for all other models the iOS update supports.
We seen this with iOS 5, iOS 5.0.1 and I guarantee you we will see it again with iOS 5.1.
Issues with iOS devices updating had a lot to do with the strain on Apple's servers. In addition to AT&T i3GS and 4, they also had the Verizon iPhone 4, all iPads, and 4th Generation iPod Touches getting iOS 5 updates from them. That's par from the course when you have SO MANY devices pulling data from your servers.
The fact that they were able to get through it at all is impressive, IMO.
In any case, the model argument is mute and has been since Apple started keeping the other model on the market as a mid-range offering, and especially since they started putting out CDMA devices with major internal differences from the GSM version.
Also, Android updates for Nexus devices are typically fast, especially bug fix and security updates, and Android OEMs have generally outperformed WP7 OEMs as far as getting updates out even on the same carriers.