The best bag fill depends on how the bag is to be used. Basically, the reason for heavy sand is to keep the bag from moving around, when the rifle is to be shot in a manner such that the shooter will not be contacting it while shooting, that and because benchrest rules specifically disallow shot or metallic substances. If you are holding on to the rear bag as you shoot the rifle, this is not really an issue, and ordinary sand will work just fine.
The other issue that is involved is the need for there to be some level of vibration damping in systems designed to support CF rifles while they are being shot. This is particularly an issue when a light rifle is shot free recoil, and much less so when the shooter is in full contact, but in either case, it is not good to shoot with the rifle supported by a hard surface.
Recently, a couple of top level shooters have told me that they have switched back to leather front bags,and that they fluff them up between matches so that they will not become so packed down by use that they have unexpected fliers caused by too hard of a front support. This can also come into the picture with rear bags.
You may also want to know that different materials have different packing and stacking characteristics, depending on the size and shape of their particles. I have seen heavy sand that was very find with relatively smooth particles that relied almost entirely on the shape of its container for stability. Other sands that had different size and shape particles can be less dependent on their container for stability.
The bottom line is that in order to discover what works the best for a given application, you may want to do some experimentation, or at the very least copy what someone else, who is successfully shooting a similar rifle, in a similar manner is using.
Also, for what little it is worth, it has been my experience that small differences in how full bags are can make a noticeable difference in how a rifle performs. This sort of tuning does not get much mention, but it can be a significant factor.