The T6, as it pertains to the 2011-16 XC70 is a transverse mounted turbocharged 3.0L short inline 6 cylinder. 300 HP and 320 pound-ft torque.
As for "which years", the newest you can afford.
This is mine:
https://farm5.staticflickr.com/4689/...f87db8ac_o.jpg
It's a 2015, and I've owned it for just shy of a year. There are a few little things that I could pick apart, but overall it is a really nice car. That's my wife's Outback parked behind it, and her old Outback was a 2002. The interior space - and the shape of that space - is better than both of those cars, and much better than the vw/audis that I looked at. I test drove the V60 wagon as well, and the sloped roofline really kills the doggie headroom. Way more road noise too.
As everyone says, the seats are comfy, and it has a fairly plush ride - yet if you're a smooth driver it handles twisty roads quite well without much body roll. The engine revs smoothly, and the acceleration will put a smile on your face. And even with all that power, the mileage is the same as the Subaru.