I feel its best to say what is one better used for than the other. Millput dries a lot harder whilst
GS dries to a rubbery consistency.
Alot of sculptors used both. Sometimes separately, whilst sometimes mixing them together.
Some sculptors make rough shapes with millput and then mold detail (muscles and such) with
GS.
Here's some sites explaining putty's.
Its a bit naughty as its from another forum but explains it really well.
http://www.the-waaagh.com/forums/?showtopic=30279
Here's a PDF on sculpting
http://www.miez.nl/putty-n-paint/tutes/sculpting_tut01.pdf
If your going to be general gap filling then i suppose go for the cheaper.
Slappy