When you add a video file to Air Video HD, it converts it to MP4. I don't believe you can play this file until the conversion is complete. Also, it creates a folder on your Mac where you will find the MP4 file once the conversion is complete.
I've messed around with Settings on both the Mac application and iOS app. I see no place where you can change this. So if you want to use Air Video HD to natively AirPlay to your Apple TV, you must be okay with also getting a converted file (MP4) onto your Mac. This isn't a huge deal (you can save the file, delete it, whatever). But with Infuse I just add the MP4 (that I already had) to my iPad. With Air Video HD you gotta wait for the conversion/transfer (happens at the same time) and also realize you have a folder on your Mac with that MP4 file. (Which isn't necessarily a bad thing.)
Like I said before, technically Infuse can handle other video formats, but it does not AirPlay natively. So what you are left with is Infuse attempting to AirPlay Mirror an AVI or FLV file to your Apple TV, and that just results in terrible playback. Native AirPlay to date for me is always smooth.
Have an iMac (lowly old 2.8 dual-core) but just for general media and internet. All the heavy lifting goes to my i7 quad core PC, including streaming movies.