Better to mixdown 2.0 or 5.1 track?

[bwh79]

For some time now I've been using HandBrake to convert video clips to import into iTunes and view on my iPod Touch. That was easy enough, I just chose the 'iPhone/iPod Touch' preset and hit start. Now that I've got an Apple TV as well, I've become interested in adding some of my DVD movies too.

I get that the 'Apple TV 2' preset makes two audio tracks, one a straight passthru of the raw AC3 audio track from the DVD for devices which support it, and the other is converted to Dolby Pro Logic II (which I understand is 2.0, correct?) for playback on devices that don't support AC3 Passthru. Some DVDs have only a 2.0 sound mix, and some have only a 5.1, so those ones are easy. Sometimes, however, there are both 2.0 and 5.1 mixes available on the disc.

I don't currently have a 5.1 setup, but I'd like to have my collection somewhat 'future-proof' in case I do have such a system in the future, I don't want to have to reconvert my movies to make use of it. With that in mind, I believe I should use the 5.1 track for the AC3 Passthru track, when it's available. My question is, when both 5.1 and 2.0 tracks are available, which one is better to mixdown for the Dolby track?