re: 11" vs. 13" vs. ??
It's interesting how perceptions vary with these things.... I was shopping for an Air for my wife earlier this year, and went back and forth, back and forth, debating on the 11" or the 13" version.
After something like 45 minutes, I concluded the 13" made much more sense all the way around. Basically, you were getting a slightly faster processor, a display with much better resolution, more battery life and an SD card slot with the 13", all for not that much more money.
The 11" Air reminds me much more of Apple's direct response to the cheap PC netbooks out there. It definitely feels a lot smaller than the 13" when you alternate between the two units, but I think after a few days of regular use, you're going to get accustomed to whichever one you go with and the other will seem "too big" or "too small" by comparison.
If you remember back when Apple sold the Powerbook G4 notebooks, they offered a 12". Maybe that was really the ideal compromise in size?
I agree with the comments towards the original poster though, that he doesn't sound like someone who will make any real use of the extra computing power a 15" Macbook Pro would give him. His complaints are purely disk storage space related (with a side concern about the fans cranking up high and the machine getting rather warm when driving an external display -- which may be running at a much higher resolution than the native 11" LCD, explaining the extra demand on the computer).
I'd probably upgrade the SSD to the largest one from OWC he can afford and call it a day.
I've had the 13" 2010 MBA and both the 2010 and 2011 "ultimate" 11".
There's substantial difference in day-to-day use. While the footprint doesn't seem to be markedly different between the two, the 11" is a more useable machine when the portability factor is taken into account.
I know others have expressed difficulty with the small screen; it has yet to be an issue for me. The resolution is superb and the full-screen feature in Lion minimizes the amount of scrolling I do.
After using the 11" for a few weeks, the 13" seems positively huge by comparison. For almost everything I do, the 11" suffices.
I travel a good deal, and enjoy not having to take the 11" out of the bag for airline security.