I was in the exact same position as you going into this school year. I had a 15" MBP which I found to be too heavy and not portable enough for class. Therefore, I sold it and took that money and purchased a base-line 21.5" iMac. Then, I bit the bullet and bought the base-line 11" MacBook Air with 4GB RAM. My reasoning was this: I thought having two laptops was extremely redundant. Especially when the current MacBook Air is actually more powerful than my 15" MBP was.
As far as model for MacBook Air, I believe the extra power and screen real estate would be nice if you have software that is demanding for such things. A 13" MacBook Air seems ideal to me, especially if you have a powerful desktop at home.
Therefore, do you absolutely need to have portability for your main computer? If not, consider a base-line iMac and an 11"/13" MacBook Air. That way you have power at home and power/portability on the go.