Just to add color to "my requirements" - most of what I need it for is word processing - advantage larger screen. Next in use frequency, research - which involves a lot of web surfing, many open tabs, side by side pages etc. - advantage larger screen. After that, streaming video content - advantage larger screen. Some light photo/video editing - advantage large screen. That right there is about 95% of what I use a computer for. The remaining 5% are things like some light data crunching etc.
Then there are things which don't need a large screen, but which benefit from extreme portability and ease of handling - things like book/pdf reading, bill pay, email, texting etc. and these are all things which are more conveniently (in my case) done on a completely different device altogether - we're talking iPad and phone.
So for me, a laptop with a large screen that's also very portable (so I can take it into the garden, for example) does pretty much better *because* of the larger screen and for my other electronic needs, I use the iPad and iPhone. In this scenario, I don't see where a 13" or 14" screen is better than a 15".
As can be seen, I don't need a ton of power, so an MBP is overkill. A long battery life is a plus, and the form factor is important - for example since I do a lot of typing, the thicker body of the MBP compared to the MBA is a negative (I wish they went back to the wedge design for the MBA, but I guess that ain't ever happening!) because my wrists are in a more flat position with a thinner body that doesn't cut into my flesh. All in all, a lot speaks for the 15" MBA over the MBP in my particular case (obviously your needs may indicate the opposite!). I could probably do with a stock 8/256 given my needs, but I figure the 16/512 is a little bit of insurance in case future MacOS versions or apps are more demanding.