I’d say shoot 1080. My impression with the 600D is that, while it takes pixels from the full height of the sensor to shoot 480, the data transfers at a such a lower bitrate that you’re losing more than you would shooting at 1080. (On the 5DMkIII, for example, 640x480 mode only allows IPB mode/10 Mbps; 1920x1080 offers ALL-I mode/90 Mbps. That’s 9 times less data for a frame that’s only about a third smaller, give or take. So it's getting more compressed. If anyone else understands this better, please chime in.)
Also, since you’ll be editing one timeline with both 1080 and 480 footage, having all the clips at the same FPS and resolution would make it that much easier. I believe without ML or firmware on 600D you can only shoot 640x480 at 50 or 60 FPS, whereas your 1080 footage will be in 24, 25, or 30 FPS. Shooting everything 1080 and cropping in post, you don’t have to deal with mixed FPS.
So I’d say go for 1080 and modify your monitor any way you can (tape?) to frame it for 4:3 while on set! Amman did say that was crucial for composing shots on ‘Dayveon.’ Good luck! Sounds like fun, would be curious to see the film when you're done.

It's from the midnight short "A Nearly Perfect Blue Sky" that's mentioned by Emily Doe below.

I haven't had a chance to see the Adam Curtis film yet, but will be sure to now. Looks frighteningly fascinating. Thanks, Ed!

Thanks Erik! So glad to hear it. There's also that scenario "they have been doing DEF for forever and I already know this is getting cut but I don't want to interrupt and mess up the vibe..."

I'm glad this is pointing you in a good direction! Please keep me posted on the film and its release -- it sounds very interesting. With a topic like hazing that could be the source of a whole awareness campaign, if that's something you're thinking about, the film could really benefit from a grassroots screening tour. Feel free to shoot me an email if you need any more advice.

Hey Joe - for every screening I have done 5 minutes before, 25 minutes after. I honestly think it's worked great to keep the beginning very brief -- and five minutes is just enough time to introduce yourself or your partner, mention the Q&A afterwards, and thank everyone for coming!