Canon C100 mk1 AVCHD vs MP4
Well first off, the C100 Mk I DOES NOT offer an MP4 option, only a AVCHD option. I'm assuming you meant the MKII as you said 'recently bought'.

If you look at the specs of the C100 MK II you'll see in MP4 mode you can't record interlaced, but in AVCHD you can (but you don't have to if you are at 28 mbps). So basically you can't go wrong (aka get interlaced files) with MP4, with that being said, you can also use the AVCHD wrapper @ 28 mbps and not get interlaced files (look out for the 'i' next to the framerates).

People primarily use the MP4 wrapper in the C100 MK II because it has the capability of recording at a higher bit rate (35 mbps) which means you have more information to work with when you get into the editing room, its good for color grading, and in some cases has less noise. I also find that MP4 works better inside of PP CC.

Also shooting in MP4 will probably not have much an effect on jello (although its never something I've experience in my C100's) but not shooting in interlaced will definitely help due to the way interlaced works (capturing every other line).

I'm a C100 MK I &amp; MK II user ;)