The difference between the 13 inch late 2011 and the 15 inch early 2011 (2.0GHz quad-core Intel i7) can be as low as $90 in the U.S. Apple refurb store, depending on specs.
The late 2011 MBPs were a pretty minor update. Unless you are a very power user, any of the early 2011 MBPs, based on refurb pricing, are a better value.
As mberdych and others have said, it depends on your needs. The 15 inch is faster and the screen is easier to view for most people. The 13 inch is a bit easier to carry, and easier in tight spaces like airplanes. If you are okay with no firewire, no internal optical drive, and less internal hard drive capacity, go for the MBA. The MBA is much more pleasant to carry around. The 13 MBA has a better screen than the 13 MBP.
For general value, the Feb 2011 13 inch MBP(refurb) is the better buy IMO. Each machine has its virtues IMO. The value equation and features may significantly change with the upcoming 2012 MBPs, expected by the summer. Its a fair guess that the Ivy Bridge chips and possibly a re-design will be a big improvement.
If I needed/wanted to buy now and hoped for better MBP features soon, I would get something more value/low end (better resale prospects) and trade up when the new generation MBP features and early track record are clear.