Hmmms.....well...they both have their advantages and disadvantages.....
Super Sculpey has an indefinite work time but for some it just doesn't feel right - like it doesn't have the strength and durability of the epoxies.
Magic Sculpt/Aves/Milliput/Greenstuff....you have a limited work time with these but the end result is generally more durable.
Super Sculpey is very easy to work with....the indefinite work time means it doesn't matter how long you take to get something right...you're not gonna waste any material. There's also Super Sculpey Firm...which, as the name suggests, is firmer than regular Super Sculpey...it's also a grey colour which helps to see detail (you can mix some black Sculpey III in with regular Super Sculpey to make it grey if you prefer the slightly softer feel of Super Sculpey).
Personally I use both Super Sculpey and Milliput - sometimes on the same figure (just remember that once you've used an epoxy on a figure you can't bake Sculpey on it anymore so do all the Sculpey work first). I also use both if I'm working on a resin piece. I bake the Super Sculpey parts separately then glue them onto the figure and then fill any gaps with an epoxy.
I find it best to use a primer as this gives you an ideal surface to paint on and as for watering down your acrylics....the thiner the paint the less obvious your brushmarks will be....but.....the thinner the paint the more coats you'll need to apply.
There's some more tips
HERE.