Re: .308 dies
First, you need to picture what these dies do. A F/L sizing die will fit the neck, shoulder, and most of the body of the case and will try to size all of these surfaces simultaneously. A neck only sizing die will size only the neck portion of the case, leaving some of the neck, the shoulder, and the body of the case untouched. A body die will size most of the shoulder and most of the body. Why most of the shoulder? Because they don't know how wide you are leaving the neck, so they have to give plenty of room so that the body die doesn't end up neck sizing.
In a bottleneck cartridge (like the 308), the case headspaces off of the shoulder, which is to say that the tolerance between the face of your bolt and the case head is determined by how much distance you have between the shoulder and the case head. As you fire a casing, it tends to grow to fit the chamber, which means that eventually, the brass will grow so long from the shoulder to the case head that you will not be able to chamber your round. If you are neck sizing only, you will need to bump the shoulder back at this point with the body die. If your Full Length sizing die is set up properly, it should bump the shoulder back every time you size it, so you never have to worry about head space issues (assuming you set the die up correctly).
The advantage of neck only sizing is that it is gentler on brass as you are only sizing a small portion of the brass at a time and bumping the shoulder every now and then. The disadvantage is that because neck sizing and body sizing happen in two steps and because the body is not as firmly supported as in a F/L sizing die, the runout, or lack of concentricity, is usually higher for N/O sizing, which generally hurts accuracy.
The best of both worlds is to get a custom die that minimally sizes every dimension with every firing. It is easy on brass and yields good concentricity. It is also the costliest route.
If you go the N/O sizing route, all you need is a N/O sizing die and a body die. If you want you can bump with a F/L sizing die as opposed to a body die, but a F/L sizing die will be a bit more costly than a body die, and generally, you can't control the neck tension in a F/L sizing die without going the custom route.
Hoe this helps.