I would prefer the DAY than the messenger as i'm not a fan of the flat messenger.....it's really depends what function u use it for as i hold lots of things and needed a larger bag.....
Day...I've just returned a messenger. Couldn't get to grips with it. No slouchiness. Strap too short. Very awkward. I would suggest trying one first, if you haven't already.
My vote is for the day. It holds more especially vs. a flat messenger. I saw the messenger style IRL and it hit me just right but was worried about how much you could get in it. I have the day and it fits a lot and is quite comfy.