I have a RTX and have been very impressed with the off trail manners and this is after coming off of a Nytro XTX and after turning that limo on the trails for three years there was no way I was going back to a long track. I am way less exhausted on my Viper after a 200 mile day.
But your question was 129 vs 137.
The difference between the 129 and 137 are actually less than 4", because they also use different size rear wheels. I think there is misconception on the track lengths. The 137 doesn't "bridge" the bumps better than the 129 and I also believe the off trail capabilities are a wash as well. The track length difference when you really get down to the math is one more lug on the trail.
IMO what people really need to do is compare features like windshield, bags, colors, shocks, ect. And decide what they like best.
Buy what you think is cool and the best bang for you buck and ride.