I got the 12.9" inch at Target the week before Thanksgiving for around 15% off (got the wifi 128gb model and pencil for around $145 off). That was the best deal at the time that I could find, and I am in need of a new ipad. My original air only has 950mb of storage left, and it can slow and sluggish at times. Plus it's my Christmas present.
I say if you need it now buy it now. It's not going to become obsolete anytime soon and just how big of an upgrade will the 12.9" receive in March? If you are happy with what you have and it is working well for you, then wait. If my Air wasn't running out of space and slowing down, I probably would wait (well I'm going back to school in January so I don't know if I could have waited).