If you already own an iOS device, Airplay hands down makes it the better choice. It's a no brainer decision. Airplay really is that good.
Since you plan to use an antenna, you can also get the awesome EyeTV software which can record shows from the antenna and export them automatically in a format for your Apple TV. You can even Airplay recorded or live TV from an iPhone/iPod/iPad directly to the Apple TV. All apps support audio Airplay and there are a TON of apps that now support video Airplay and in iOS5 Airplay is enabled by default for apps as well as websites have video viewable on your iOS device in Safari.
Previously, the website owner had to enable Airplay for the video by just adding Airplay="allow" to their code but not many ever got around to doing that. I and many others have been using a bookmarklet that yo just tap to make them enabled for Airplay but with iOS5 you don't have to do that anymore.
In iOS5 there is also now an Airplay Mirroring feature where whatever is showing on your iOS device shows on your TV through the ATV. This works great for presentation software like Keynote or anything you want to show someone else on the big screen. You can even use it for games and internet browsing.
Also, with Home Sharing all your content on your computer is available on the Apple TV even from multiple computers simultaneously. All your locally stored movies, podcasts, iTunes U stuff, tv shows, music, etc. Even rentals downloaded to your Mac, PC, or iOS device can be either Home Shared or Airplayed to the Apple TV. The Roku doesn't really have a way to play your local content. It is pretty much an internet streaming device only.
There are a ton of other cool stuff like all the hundreds of free "channels" available with the podcast feature, the many radio stations, and how with Photostream in iOS5 you can actually take a picture on any iOS device and the photo is instantly available on your Apple TV. Even before iOS5, you could Airplay any photos or movies sitting on your iOS device that you just took but iOS5 makes it even better.
Plus Apple is planning on adding an App Store to the ATV which will probably allow gaming on the device among other things. It's a pretty neat device, very small, cheap, doesn't get hot, and is completely silent. The only thing is you might want to wait until September because it is expected to be updated then with the A5 processor like in the iPad 2 which supports 1080p video.