Like the previous posters have said the main difference is rim width and profile, 2015 uses the DET design which is 26.2mm wide, the 2012 is a SLG design at 21mm.
The 2015 will allow you use wider tyres if that is your goal, it will be more aero with the wider 25mm and up tyres.http://www.reynoldscycling.com/technology/aerodynamics
In my riding of both wheels, I have the tubular versions, the Aero 46 is different feel in strong winds, there is more of a constant side pressure, for the 2012 the thinner rim is more of a twitch then a correction.
It's just something to get used to, in reality not much different. In some areas I actually prefer the 2012 sometimes as it accelerates quicker due to the lower rim and tyre weight (narrower tyre), but it's not as comfortable on rough surfaces which benefit from having a higher tyre volume that you can use with the Aero 46.
There maybe some limitations on how wide a tyre you can put on the 2012 model.
The hubs are different in design, Aero 46 has much lower profile straight pull in a 16/20 spoke configuration and the 2012 has the the older style of hub in a 20/24
Both models are radially spoked on the front wheels, with the 2012 rear being radial on the non drive side and 2 cross on the drive side using j bend aero spokes, the Aero 46 rear is 2 cross straight aero spokes on both sides
The only visible difference in the rims is the rubber grommet to stop the valve extender knock on the 2015 model.
Both have the Ctg brake surface technology
So it's really a question of which width of tyres you want to use. also you won't get the next gen cryo blue power pads (usd 65) with the 2012 model.
The new 2012's on ebay are a pretty good purchase as they are close to the second hand prices