Look at them
I went to my Apple Store last December with full intentions of buying a 12" ibook. Once I arrived though, I chose the 14". The 12" just seemed a bit compressed to me. I love my 14" and am glad I went to look at them. So thats my advice, you really do need to see them next to each other, and weight what you will use the ibook for most often. BTW, my 14" ibook's LCD is bigger than my imac's CRT (by .66 inches actually), if that gives you some perspective.