I have both iPod 5G and rMini and can confirm that both will do AirPlay well. The rMini has vastly better battery life in spite of its much larger screen - i get usually 10 to 12 hours in mixed use (not just AirPlay). My 5G iPod is usually less than 4 hours, but its battery may be getting old. (the battery is essentially non replaceable, by the way)
The advantages of the 5G iPod are its small size and low cost.
You may want to give some thought, however, as to how you might end up using both devices for things other than AirPlay. If using web content, reading Kindle or iBooks, access to a cell connection, etc. have any value to you at all then I'd recommend the rMini.