Well, nothing to see, SOL601 is really powerful and the price is rather competitive, SOL106 is quite limited in many aspects, specially in contact: surface-to-surface contact is not supported, only 1-D CGAP node-to-node and Slide-Line contact elements are supported. Also the limitation in Large Strain < 10% is really a handicap, but I use SOL106 a lot as well, is very easy to setup any nonlinear analysis and when the nonlinear problem involve contact or more complex aspect then I move to Advanced Nonlinear SOL601.
Please note SIEMENS is working since years in a native nonlinear code named SOL401 that will have soon the best of both worlds!!.
SOL106 is the legacy NL solver, dating from before MSC acquired Marc. SOL601,106 is an integration of NX NASTRAN and Adina (very much the same way MSC has done with Marc) and offers a much greater amount of non-linear ability (contact, post-buckling, element birth/death,...), to bring it on par with the other big two non-linear solvers on the market. SOL106 is decently capable for mildly non-linear problems, but really can't compare to SOL601,106... 106 comes bundled with the basic NX NASTRAN module, but 601 is an add-on...