The only time you dither is when going from a higher bit depth to a lower one, usually from 24 to 16
There is not a lot of difference between Entire Mix and Main Outs, but this comes with a warning:
If your interface has more than one set of Stereo Outputs and you explicitly set up a 2nd pair (or more), say for headphone monitoring, an extra 6dB will be added to the bounce (or export) for each additional pair.
Put another way, using Entire Mix will SUM all active outputs into the bounce which is usually undesirable.
Using Main Outs will only use a single stereo pair, maintaining the volume of your mix.
I only ever use Man Outs for bouncing/exporting.