I could afford a MPB without worries, but in the end I got a white MB because:
* My primary reason for getting a notebook is so I can work on my projects wherever I want: on a lap desk in an easy chair, out in the backyard, on the road, etc. I value small size and low power consumption.
* The other big difference, besides screen size, is the fancy GPU in the MBP. This makes no difference to me performance-wise, but it generates more heat and decrease battery life. I'll pass.
* I think the MB is more aesthetically pleasing overall.
* This is a minor thing, but the power brick for the MB is smaller than the MBP. Again, less to lug around on trips.
From my point of view, both machines are complete monsters in terms of performance. I did commercial software development on a Power Mac 6100/66, a 400MHz Pentium II, and an 800-ish MHz Pentium 3. GPU aside, the PC I have on my dekstop right now (hot-running single core P4), which is used for very high-end software development, is slower than the MB. That I can have what amounts to two high-end processors and 2GB RAM in a notebook still blows my mind.