You're asking about two very different bags here.
The Lockit Horizontal is HUGE - 17.3" x 12.2" x 5.1." It would be fabulous for carrying books and papers, and it's the only Lockit you can put over your shoulder. You could probably even fit a small laptop in there.
The Damier Speedy 25 is a great everyday bag (it's my current one) - goes with everything, from casual to professional to dressy. I'm 5'7" and the 25 is a great size for me - I think the 30 looks too big for its handles, but that's just me. It can hold a few extras, like maybe a paperback book or a sandwich, but there's no way you'd fit textbooks and stuff in there. So it depends on which you think you'd use more!