I'm working a little bit off of memory here, so someone can correct me if I'm wrong, but as far as I know, the Outback has not had any major recent changes. In fact, it's one of the older designs in the kayak fleet.
The Revolution is a bit newer, I believe it was new in 2008. The Oasis was new in 2011, the Revolution 11 was a 2012 boat. The Adventure Island and Tandem Island got the rudder redesign over the past couple of years.
Boats like the Outback, Sport, and Outfitter are all slightly older designs with no major upgrades recently.
So the catch becomes... if you buy one now, how soon will your model be the previous generation model?
But seriously, a lot of people are very happy with those boats, so if you like it, there's no need to wait for a new model that may or may not be coming out in the next few years.
The Outback has traditionally been the "fishing boat" of the kayak line but it seems that Hobie has put their fishing expertise into developing the two Pro Angler models, and haven't done too much to the Outback in that time. Why change what works?