Easy to use Netflix UI, AirPlay, and access to the iTunes store is the main reasons why I got an Apple TV.
AirPlay is amazing when you need to show something quickly from your iOS device. It literally takes two seconds to mirror the screen from my 4S so I can show something or just send the video/audio to the AppleTV.
It's a $100 device, if you didn't have the other devices, and you're already in the Apple ecosystem, then it should be a nobrainer. But since you already have a PS3 you can hook up that is a little more platform agnostic, it might not be as worth it.
If you already have a Mac Mini that means you're heavily invested into iTunes and likely have much of your media organized. I find iTunes extremely frustrating to constantly organize and so I primarily use a PS3 to play downloads and Netflix. Yet, I do not have a dedicated HTPC and am considering an Apple TV for it's portability to use downstairs and/or the bedroom to see just how well I can port my media into iTunes.
For $100 Apple TV does a lot when you consider Airplay and transferring your iTunes "cloud" around your house. I think it may be worth it for you since your can't share your downloads from Mac Mini to PS3 as easily as Apple TV.
Sound like you answered your own question. If its just as easy, then for sure go for the PS3. It also has other nicer features like a web browser.
Only if you are concerned about size should you opt for the apple tv in my opinion
So AirPlay is definitely not a selling point?