I have been using SR3D Builder to create my digital models after finish my real models from more than 5 years.
I use it because in my opinion it is the best software to create technic models. You can do more things and it has more parts than LDD, and it is easier, more intuitive and quicker to build than MLCad.
Also its 3D models files are compatible with all LDraw tools(MLCad, LDview, etc).