Milliput behaves very differently than GS. It's good for bulking out areas, terrain, bases or as a wash for blending the transition between sculpted areas and the model itself.
Milliput is very cheap in comparison to other products and comes in a variety of grades for use on different things.
I personally would recommend Procreate/Grey Stuff as your baseline sculpting medium. It is easy to use and reasonably cheap so it can be used for bulking out areas without destroying your bank balance. I use a range of putties for different jobs (Green Stuff, Brown Stuff, Procreate, Milliput), but then again, I do a lot of sculpting so want the right tool for the job. If you just want one putty that can be used for all jobs then Procreate is good enough.