I recommend you to see tutorials online, maybe youtube. There are some on coolminiornot.
For every material there's certain learning curve, and as you said, certain applications use their capabilities better than others, but with just those two, you got some of the best materials for miniatures available, and the easiest to work with (you don't have the odors of the waxes, spilling the alcohol or burning yourself)
You shouldn't discard them. They both are the standard materials for 28 and 54 mm fantasy, military miniatures, and will give you more details at an ease of work than many polymer clays or other epoxies.
Green stuff isn't easily tooled/sanded, as it's really flexible. Milliput cures rock hard and is sandable. Mixing them gives you both, certain elasticity and hardness. You also could have variations in hardness mixing green stuff with the hardener in different proportions (the blue putty). I always prepare different mixes before sculpting for the different zones I need.
I've achieved hard edges and lines, specially with milliput. Normally everything is done in layers, so you first make an approximation, rough sculpt, then sand the excess and add another layer of material. After time passes, in both putties, you get some change in their consistency, that allows different things to be made, waiting at the end of the curing time allow you to do fine lines.
If you mix them with alcohol, you'd get it to be chewy. If you use an oil, they'll be really easy to handle, but won't stick to anything until you deoil it's surface. You could use a heat gun to speed up their curing.
The mold materials you selected is one of the best, easy to start with too.
Cheers and post pics of your sculpts to see your progress