Most HH motors can be run up to 150' using standard RG6 coax. Longer lengths many require a better quality cable such as Quad RG-6. The distance that a motor can be controlled is more a function of the receiver that is controlling it. Some receivers put out a strong DiSEqC signal, while other receivers are weak.
If you are burying a coax cable, it should be labeled "Flooded" or "Direct Burial". Regular coax is not suited for this purpose and the cable will most likely fail or have degraded with constant exposure moist soil within a few years.
The GS120 has high quality hardened steel hardware, simplified installation procedure and has been USALS compliant for several years. The new SG2100 has changed to a two button design upgraded to hardened hardware and is USALS compliant. We have opted to stay with the single button design as we found during testing that this design is more reliable and easier to reset. Both are fine motors and I am sure that you would be very happy with either brand.