I now have both because I drove myself crazy debating between the 25 and the 30. Unless you need to carry magazines and such, I would say the 25 is large enough to carry everything you need. I can fit a spare pair of shoes, a water bottle, camera, lotion, tissue packet, wallet, keys, cell, scarf, a makeup pouch or two, and still have room for other little things. The 25 is small enough to be carried to dinner whereas I feel the 30 is too big to be a dinner back. Mostly, I like the 30 for days I just want to look chic, or as a carry on bag when traveling, long day trips where I wont be annoyed having to hand carry a bag, when I want to carry a magazine, when I want to carry a lot but don't feel like having to "arrange" my things so it fits (the larger opening helps). The 30 fits A LOT, I can fit 5 pairs of rolled up jeans. For all other occasions, I like the the 25.