You can mirror presentations and demo apps with the AppleTV as well. The only difference between the 2 choices is price and wired/wireless.Depends what you want to do with it.
If you just want to mirror the multimedia aspects then either AppleTV or the AV adapter. AppleTV would be the better choice & you can use the Remote app to control the AppleTV on your iPad too.
If you want to mirror the iPad itself so you can show presentations (like keynote) or demo an app on the iPad + all the multimedia stuff then you want the AV adapter.