I graduated school in december. Over the 5 years I was in school, I had a 15 inch PowerBook, a 12 inch iBook, a 12 inch PowerBook, and two MacBook Pros. The 12 inch powerbook and iBook were my favorite class computers. I worked on them at school, and went home to my G5 with a 20inch display. However, when I sold the G5 I just had a 15inch MBP and my 20inch display, that combo seemed to work the best.
My recomendation is to get a 15inch MBP, and with the money saved get a 20" Apple or Samsung display. The Samsungs can be obtained inexpensivly at newegg, and look far better than a dell monitor. However, I sprung for the Apple display, it looks awesome, and it matches the computer🙂 .