I've tried quite a few of these epoxy putty over the years.
Starting with Milliput. Havent tried Apoxie. But I've converted to using Magic Sculpt which I bought at Euro and it's great! The main difference would be that Milliput tends to be a firmer mass and if you wet it to smooth it, the surface tends to melt into a mush. Magic Sculpt seems softer to me and gets firmer as it sets. It has excellent blending properties.
These days I've also started using Kneadatite (Blue and Green stuff) which is really firm when soft and a bit rubbery when set. I mix it with ordinary plumbers epoxy (bondite, araldite, etc) which is really soft. This way I can control the firmness based on the mixing ratio. The knead doesnt sand very well cause of its rubberiness when set, but mixed with the bondite (which is really hard when set) you get a really nice material for carving and sanding. The only problem i see is that blending is a bit tricky.
And as a finishing putty, I use tamiya putty thinned with enamel thinner. Perfect for filling up very small cracks and blemishes.
So my advice (though a bit pricey) is to buy a little bit of each of the different putties to see which one suits your working style. Or if you just want to get one - I recommend Magic Sculpt. (I'd like to try apoxie though)
