If you plan to keep the machine > 3 years
- If budget is limited, go with MBP. You can upgrade RAM, disk, etc later
- If budget is no issue, go with MBA but upgrade the RAM (must) and also disk (optional)
If you plan to renew every 2-3 years -> MBA. No point upgrading MBP later, because you will get the new generation anyway. Exception, if your usage requires > 4 GB RAM
Bear in mind, You can't upgrade processor on both. 3 years old machine usually start slowing down. The difference is not noticeable if you upgrade ram/SSD in MBP compared to MBA
I used to have MBP for 3 years, and found that even upgrading SSD and RAM didn't help much on performance (still core2duo). I sold it for MBA 128/8GB.