AA you gotta know how to use your tools. Ozone is like a bunch of plugins put in one plugin with presets and some extra eye candy. If your happy with your daw then ozone might be your answer, but AA has a lot more to offer for a extra buck.
I honestly love Ozone 4. I use it on all my songs. Yeah, there is a bit of learning curve involved with Ozone and I hate all of the presets but when you learn how to actually use it, you can do some magical stuff.
I'll never master in adobe audition again. One time I was using the parametric eq in the mastering rack and i accidentially typed an extra 0 for the gain on one of the eq bands, and this shit went +78461478670db and it didnt limit it from going that high. Mind you i have the audio at -15db and my speakers loud to compensate. I had to use my elbows to hit mute on my mackie bigknob because id prolly go deaf if i didnt cover my ears.
Simplest - Ozone since it puts your typical mastering tools together. As stated above with AA you have to know what tools of theirs to use then on top of that you might still end up needing/wanting 3rd party plugins. If you already own AA try it but if not go for Ozone.