Upgrading OS X has never resulted in my system using fewer resources. New features often result in extra processes and daemons running in the background. After a restart of 10.6 Snow Leopard, my machine would have about 30-40 processes running in the background. Now in Yosemite PB2, I have 184 processes running in the background after a fresh restart with Mail and iStat Menus launching at startup.
The aggressive caching that OS X does nowadays really takes its toll on a machine with a spinning disk, causing my new machine to feel a little more sluggish than my old Core2Duo MacBook. With the much faster read/write speeds of a machine with an SSD, such as your MBA, you'll see less of an impact. My new SSD and ram upgrade should arrive in the next couple of days, and I can't wait.
With all of that said, I see little difference in the way my machine runs, particularly the ram pressure and usage, in Mavericks or Yosemite. I imagine that by the time Yosemite reaches its production release, resource usage will see at least slight improvement over the beta releases.
If you machine handles Mavericks to your liking, Yosemite will likely be just fine for you as well.