This a.m. I was out with a large tote and had a sweatshirt, son's shirt, wallet, two checkbooks, novel, papers, wristelets, etc. all in my bag. As much as I like the look of the 25, it probably won't be as practical for me as the 30 will.I had a Speedy 30 first, and then I added a 25 later. But, the 25 was too small for my needs, so I sold it. I carry alot of accessory bags in my Speedy, so the 30 size is great for me. I'm 5"3' and I thought the 25 looked too small on me.
Do you carry alot of stuff with you? If so, go for the 30. I also was able to stuff a small thin jacket rolled up into my 30 as well as my water bottle when I was walking around SF one time and it was great! I also recommend getting a strap for your Speedy.
The 25 holds way more than it looks. Believe it or not, that is one bag I grab when I have a lot of stuff to carry. I have shoved sweaters in there! It's very deceiving.