My wife and I have identical 2013 13" MBAs. Mine is on Mavericks. Hers, which was bought a little later, is on Yosemite. So, as close as possible, we've got identical HW with different software.

- My battery lasts longer, maybe 2-3 hours more per charge. I nearly never see < 50%, she's hitting 10% at the end of the day. (Note that both are good enough, but they're way different) - Mail search sucks on Yosemite. Today, we were searching for subjects that I know are there because I'm looking right at them, and nothing is coming up. - Safari does something funky with process per tab that manages to orphan process that take a bunch of cpu/memory. - Time machine is really touchy about backing up to the server over wifi. The last few times, it's had to make a new backup on the Yosemite machine. Mine misses a backup every day or so, but that's just an hourly miss, not a full rebuild.

We're considering blowing away her machine and pushing it back to mavericks with Time Machine/fresh install.