I cannot offer very deep experience
it is long ago that I did lead conversions, and I used Milliput to fill the gaps
recently i done some very little modelling (saddles) with greenstuff, and though good for its firmness in modelling, I find it very hard to work with it
I also heard that modellers use any kind of modelling putty for different purposes, up to fimo
so I guess it comes to what the more experienced said:
it seems to be a personal preference, and I admire anyone who really is into the hard work of developing the skills to create the wonderful miniatures we all buy or who manage to convert them to that standards
so good luck and share please
I personally do a lot of conversion in plastic, but this is for the lazy one and not the topic here