My first RM bag was from a sample sale and I got the brown/chocolate MAB. I carried that bag the first couple of weeks and had thought to myself that I wish the bag were a tad smaller since I don't need it to carry clothes around, just work stuff and everyday items. Well fast forward to a couple of months later and joining TPF and I find out there is a smaller version called the morning after mini. I had to get one to compare and I am very satisfied with both styles. Someone had posted that the MAM was similar to a LV speedy 30 which I'm familiar with and its true only difference you can wear the mam on your shoulder and it has pockets both inside and outside the bag. I still keep both bags in rotation but would prefer the MAM over the MAB for everyday use.
I notice the difference but I heard though it depends on how much you put into each one. Personally I find the MAB to be heavier and it looks more overwhelming on me although some girls really like large bags and say the actual weight of the two do not differ by very much. There are some other threads dedicated to this so you might want to do a search. Good luck.