Q: Whats the difference between the Apple MD761B/A and the Apple MD761B/B models?

From what I can tell it is specifying the amount of flash storage that is available in the computer. /A is 128GB and /B is 256GB.

Actually, the difference is the year they were made. MD761B/A is a 2013 model, while MD761B/B is the 2014 model.

This means that the MD761B/B model has the newer generation i5 processor, which is slightly faster.  1.3Ghz vs 1.4Ghz

Storage space is identical on both. 256GB.

How to identify your MacBook Air - Apple Support

OS X 10.10 Yosemite is the latest version of OS X for the MacBook Air.

Tiger is a very old Operating system. Its likely you can update it to Yosemite when you get the computer, Yosemite is the newest operating system, and has all the latest security patches and fixes. Tiger is no longer being directly supported by Apple.