The baby Paddy is good for day if you don't have big files to carry and what have you. I use it when I go to the office and it fits up to a book, PDA, phone, wallet, coin purse, keys, fountain pen, lipstick, mascara, pen, transport pass and office keys/passes.
But it's really a question of what you want to carry. If you're able, go try out the different sizes and see which one you like best. Take your usual "load" and ask if you can try to fit it all in the bag (my SA let me do that the first time and it really helped my choice)