There will be more aftermarket support for the 15s than the '09-14s. There are lots of these around both in the US and even more globally. They aren't rare vehicles by any measure and VW has already demonstrated that they (VW) aren't going anywhere.
The 2015 is a much more refined powertrain...they had years to address the niggles. For example, I'm much less worried about the electronic crap than the HPFP.
One caveat is that I moved from a JSW to a GSW, so I got an entirely different chassis. I don't know the ins and outs of the Golf platform so I can't really speak to the differences between the -14s and the MBQ. But you already know the award winning Golf platform for many years.
BTW, are you worried about powertrain stuff or body panels? We share everything with the gassers except the TDI stuff, most of which is covered under the diesel gate warranty (and in the case of the CPO it's unlimited mileage). I don't know how far in the future you're projecting owning this car or what kinds of issues will arise, but parts will be available and gurus will still be around...there will still be a whole new generation of gurus when the old guard retires
My bottom line is if you're worried about the 2015, you should be more worried about the 09-14s. And if you're worried about both, you should be worried about diesel in general. And if you're worried about diesel in general, then we should be talking about getting you into a GTI (or Mazda or whatever people do when they leave the dieselclub). *But* if the GTI doesn't appeal to you, we should loop right back around to the 2015 TDI.