Really depends on what you're going to use it for.
I dual boot on my MBP to do occasional gaming, so I'm glad that I have a MBP. I can play recent games at relatively high graphics settings without any problems. At home I also have three external monitors that I hook my MBP up to; this again either wouldn't be possible or would significant impair performance with an Air - assuming the Air even has the necessary ports to do this. This wouldn't be possible with an Air. If you use your computer primarily for stuff like checking e-mail, web browsing, and watching videos, then the extra hardware of the MBP isn't necessary.
I don't think the MBP is hugely heavy or a huge burden to carry around, so the "advantage" of a smaller and lighter form factor in the Air isn't a huge deal to me. I'm glad that I ended up going with a higher end MBP; it's approaching 4 years old now and can still handle just about anything I throw at it with ease. Unless something tragic happens, I bet I will be able to at least get through residency with this machine, and I bought it at the beginning of MS3.