A few comments after reading the above -
Expect to get pretty similar battery life to what Apple quotes in the specs. They tend to be pretty honest with those specs though obviously battery life will vary based on what you're doing. My wife has a 2012 '11 MBA with the i5 (and 8gb ram) and it gets an easy 5 hours using MS word, several safari windows (including 1-2 with streaming audio and sometimes video), and syncing to dropbox every few minutes. The 13 inch does have a bigger battery with a couple more hours of life.
Speaker wise, I cant compare the 13 and 11 as we only have the 11, but the 11 inch speaker is a bit tinny. I don't mean to knock it b/c you do have to be realistic as to what to expect from such a small/thin laptop. It is 100% sufficient for the webex (audio/video conferencing) she does for her online classes. Music would not be all that great, you'd prob be better off plugging in headphones. And in a noisy environment, it
may be hard to get it loud enough to be clearly audible; again headphones would prob be your friend here.
Having said all that, the 11inch is a great computer. We also have a 2011 i5 imac with 16gb of ram, and her MBA doesn't seem (for regular tasks, I've never used aperture or imovie with the MBA) any slower at all. The SSD and 8gb of ram do wonders in overall speed. And the size is awesome; it fits easily in just about any of her purses (then again, our imac could fit in some of her purses
) and it is great when using it on an airplane tray table, your lap, etc.
I dont think you can go wrong with either. If it were me and it was my only machine (and I wasn't using an external monitor ever) i'd prob go with the 13. I used a 13 inch macbook for my prior 4 laptops and it was great. In our case though it was just a second computer she could use on the couch/traveling for school so we went 11. Good luck and enjoy the new computer!