I replaced a 2008 15" MBP with a 2011 13" MBA (1.7ghz i5, 4gb RAM, 256gb SSD). The MBA runs circles around the old MBP, about twice as fast for CPU bound tasks and much, much faster for anything involving disk.
I am doing fine with 4gb running pro applications like my old versions of Final Cut Pro, Photoshop, VectorWorks and the newest version of Logic Pro. I would have gotten 8gb RAM but that wasn't an option in 2011, and really I am not seeing any problems with 4gb. I can't imagine that you would need 16gb for what you describe (although extra RAM isn't bad
). I would put the money into a large SSD myself. 256 was the largest available in 2011, today I would go for 512.
But the 15" MBP is a very different animal from the 13" MBA. You need to decide what fits you personal needs first based on size, weight, screen, additional ports, internal DVD drive.
Personally I didn't mind giving these up for the MBA. I still use my old 15" MBP to watch DVD's, but that's about all.