I personally cannot afford the 15" and don't think it's very portable especially as I will be carrying it around collage. May I ask why you think the 13" is poor value for money?
I'll attache a screenshot of the comparison just to help out, but basically, the price difference (for me) was £283.20 - (lets call it £280 for simplicity's sake)
So for that extra £280, you're getting:
- A dedicated NVIDIA GeForce GT 650M 1GB GPU (in addition to the Intel HD 4000 integrated graphics), capable of powering three external displays (the 4000 can only do two, and seems to struggle on occasion)
- A 2.3Ghz Quad-Core i7, with Turbo Boost to 3.3Ghz (compared to a 2.5Ghz Dual-Core i5 with Turbo Boost to 3.1Ghz) - This will allow for much better performance with the likes of video conversion, software compiling, etc
- Double the SSD (256GB SSD on the 15", and 128GB on the 13")
- Higher resolution (not really a pro however, but notable if you like having a fair amount of screen real estate)
After writing this, it has to be said that people buying the 13" rMBP seemingly have no reason other than for the retina display. The 13" Air would surely be a much better choice if you're after a 13" Mac. Sure, it's using a slower CPU, but if you need a powerful CPU, you'll be either gaming or using pro apps, at which point the 15" makes more sense due to its GPU.
If its any help, I'm coming from a 2011 11" Air - I've played Portal 2 on it with no problem, I've used Xcode for iOS dev, done some light compiling - it handles all of it fine. The 11" and 13" air's are great machines and theres virtually no difference between them other than 0.1 of a Ghz and the screen size.
Go with the 13" air - it's a better option (based on your needs) than the 13" rMBP.