Here's my two favs. The longer handled one has seen nearly 3 decades of service to me. The shorty I got from Gerry Beranek, and it's something I treasure for that reason alone. It works might fine in brushy and otherwise tight swinging conditions. Both are 5 pound heads.
Those are the rough lengths of our Parks axes, though the shorty is not 5 pounds.
Our situation is somewhat unique in that its got elements of residential work and logging. Structures and infrastructure that you don't deal with in logging, along with bucket access, with constraints sometimes similar to logging, having to get wood into a tight lay, and lots of trees in a day. Sometimes we have pulling machines, but also, its not practical to set pull lines every time, even when we do have a machine, so wedges, and occasionally jacks (which we normally don't carry on the truck, but will, once we get our new jobox installed). Many are smaller trees, say under 36" and around 100', and not so bad to pound over, but sometimes larger 4'-5'-6', and over 150'. Sometimes, no wedges are needed, but often some pounding is required, as the lean and lay don't always co-incide. Putting trees back into the forest instead of into developed areas like parking lots and campgrounds means these dead and or rotten trees are left to cycle back into the forest. We try to wedge the back into the woods as much as we can, as its not feasible to bring them to market, even if they might be marketable, which they usually aren't.
momentum = 1/2m(v)2 that is one-half x mass x velocity x velocity
Getting the ax/ pounder going faster is the key, but if its possible to only swing slightly less fast, there can be more momentum with the more massive pounder.
Also, as I recall learning on trail crews, heavier sledgehammers counter-intuitively led to less wrist problems for breaking rocks for rock work projects like drains and retaining walls. Somehow less vibration is what I was told.
This question has come up before, and fallen by the wayside. I recall the deadblow being suggested. We may give it a try.
Never used the magnesiums. We have one that is the hammer shaped one, but only that one. How do they stack? Generally, if we aren't needing to stack, the trees aren't that hard to pound over.
We are also revisiting the plates for stacking with a single wedge, in which case, it may not matter how the mags stack. How are they for deformation? What type of plastic is best. How about some wood put through a thickness planer? Flat sawn maple or oak?? Tight grain old-growth fir??