I have worked with Magic Sculp, Apoxie and Tamiya epoxy putty... they all great to work with. I'll give you my mini review...
With enough water Magic sculp can turn a bit muddy... so it's easier to fill holes without sanding. Very easy to sand when it's fully dried cos it's rock hard!
Tamiya Epoxy putty is very sticky to work with...I find that I always need to apply water all the time otherwise it'll stick to the tools. It doesnt turn muddy like Magic sculp, this material is more in a solid state most of the timeand rarely you'll see breakage so it's great to make hair and other complicated parts eg. sculpting mechanical parts etc.
Apoxie is what im using most of the time now... great for general purpose modifications. Easy to apply and work with. It doesnt turn muddy like Magic Sculp but i find it doesnt dry up rock hard like Tamiya or Magic Sculp.
It's great stuff. I'd also recomend for making hair, looking into Kneadite. It's a two-part epoxy putty, you need to make sure you keep vasaline on your tools while working with it, as it's very sticky, but it's great for sculpting hair. It's also known as "green Stuff", as it comes in a yellow and blue parts you mix together.