I run both x12 and s120LT for months, so far I only damaged the stock Alu shaft on 120. The rest of the parts are all good.
In my own driving style, I like 120 more as it can attack in corners where I can punch more without spin out using similar tires, traction compound, esc/motor and rollout.
I bought the new graphite shaft for 120 but yet to test.
One more note, fit max balls on diff if you can, and then run in the diff about 2 mins before run, it makes a big difference.
I ran XII Link since 2010, overall happy with the performance but X12 is a lot easy to setup and and more stable - design of X12 and S120 has wider front wheel base compared to the previous layout.
Another difference is the balls in front suspension - XII use metal ball and X12 use plastic ball. Plastic ball allows smoother action.
I ran in track in Singapore, usually hot weather.
I used SpeedMind F 35 and R 30 tires (red box) recently with Ground Effect or FX II additives.
Build and setup the ball diff normally, just tight enough and it is not skip.
Throttle 1/4 to 1/3 when holding one side of the rear tires, do it slowly, don't gun too much, speed can be vary from time to time, for about 1 min.
Check the diff, either tighten it or loose it a bit, make it just not skip.
Run in for 1 more min.
Repeat until u find the diff is very smooth, and not skip.
Hope this helps.