My first suggestion would be to try and not listen to guy's who are giving their opinion without knowing anything first hand.
There is a significant difference between 12.6 and 12.7. I have recently just gone from the 12.6 to 12.7.
Build wise the 12.7 arm base and collar are now each made from a single machined piece , greatly increasing the rigidity. the VTA tower is more precise (not that there is anything wrong with the 12.6). All this adds up to a significant increase in bass depth and dynamics.
The 12.6 arm wand is undamped, where-as the new 12.7 has a damped bearing and has gone to a twin wall construction with superior damping and has a slight change in headshell geometry.
Overall the 12.7 arm is a fuller sounding, and more coherent arm top to bottom. The bass definition and power is increased, while it still has great transparency and life. Overall easily the better arm.
If your system is warm sounding, the HRX with 12.6 will do just fine. The 12.6 with HRX can sound a little lean. If your system is neutral or lean sounding the 12.7 would definately be the better choice.
RE your question re tinkering. If you use the air bladders you will be tinkering every two weeks by blowing them up. If you use the ball suspension, there is no tinkering required after you have set up the table. The beauty about the JMW arm is that adjusting VTA is so easy, you actually end up using it often and it is no hasstle.
cheers, good luck