i am personally trying to decide between a 30 and 35 at the mo!
i have two 25s and lurrrve them to bits, but i find in the winter it is a pretty snug fit, due to the fact that i am often stuffing extra tissue packets and mitts/hat in there. i don't know where you're from, but i think that seeing as it is now winter, i would get the 30 first, and then get a 25 for your next bag, as in the summer (at least i do) one tends to carry less stuff
i have two 25s and lurrrve them to bits, but i find in the winter it is a pretty snug fit, due to the fact that i am often stuffing extra tissue packets and mitts/hat in there. i don't know where you're from, but i think that seeing as it is now winter, i would get the 30 first, and then get a 25 for your next bag, as in the summer (at least i do) one tends to carry less stuff