I've owned both. Air-cooled GS was replaced by MTS Touring (Ohlins ESA and ride by wire modes) 2 years ago.
I find it hard to say which is "best" - they do different things really. The MTS is essentially a sports bike in an upright frame. It's way quicker and much more agile than my GS was but the GS is much more versatile. I suspect that if you threw the MTS down the road, it would dissolve!
The MTS has been unexpectedly reliable. Heated grips died at 18,000 miles and I had one episode with the keyless fuel cap refusing to open. Otherwise faultless.
I've ridden the WC GS but only briefly. I liked it - clearly more lively/refined that my old one- but not enough to tempt me away from the red side. Interestingly though, I also tried the new MTS with the Sachs Skyhook suspension and twin-spark engine and that couldn't tempt me away from my Ohlins version.
The one thing that I think is an issue on the MTS is the ergonomics. There is no adjustment, so if it doesn't quite suit/fit you you're into after-market bits. I fitted lower pegs and changed the screen for a bigger one, as the weather/wind protection from the OEM one is a joke.
In the real world, on a run or touring, it really just comes down to which you prefer. A well-ridden GS will keep up with almost anything, so the extra pace of the MTS is more about how it feels. Next on my list to try is new new GSA...