Mathematics, MBA 11' i7 8Gb or MBP 13 i7 8Gb SSD?

Well, for applications that do a lot of number crunching (e.g. matlab, photoshop, don't worry about stuff like Skype), you'll want the best processor, so upgrading to the 2.0 GHz i7 would be good, and 8 gigs of ram would also be good. But, from your description, it sounds like that what you are doing anyway, so I'm not sure what you are trying to compare or ask with the 11 vs 13"? Is it just the size of the screen? If so, I would consider how portable you need the machine (11" is better obviously), but, the 14" will give you more space on the screen, which is much more useful when doing lots of work, so based on that I might suggest the 13. Try comparing the sizes of windows you can use in the 11 and 13 and you'll see what I mean...

EDIT: sorry, I just realized you meant MBA vs MBP, apparently I am illiterate! Well, there's really 2 factors to consider then: portability and power. The MBA is obviously much more portable. The processor on the MB Pro is much more powerful though. Do you ever do analyses that take a long long time? If so, I would get the macbook Pro. That's where you'll see a big difference. And by "long" I mean many minutes, or hours, or more. If your time is valuable and you do long analyses, the better processor will make a big difference. Otherwise, get the MBA for portability. I used to do very long genetic analysis (pure number crunching), and we had a powerful PC, but it would literally take a WEEK of processing to do this, eventually we got a powerful mac pro, and the was cut down to a few days, because of the processor. So, it depends on how you are using it.

I think what you choose is down to what you require from the computer.

What is your primary requirement?

Portability: 11" MBA > 13" MBA > 13" MBP.
Screen Resolution: 13" MBA (1440x900) > 11" MBA (1366x768) > 13" MBP (1280x800).
CPU Performance: 13" MBP (2.9Ghz i7) > MBA (2.0Ghz i7)

If you make use of the GPU for your computations because of the way Intel's HD4000 works you would get slightly better OpenCL performance from the MBP. However, if you make heavy use of the GPU for computation, then the discrete nvidia GPU in the 15" MBP would be a much more powerful option plus you would have 4 dedicated CPU cores instead of the 2 on the MBA's and 13" MBP.