So the M18 fuel Hackzall is the clear winner?
NO such thing. There is a M18 hackzall, but not Fuel.
I have owned the non-Fuel M12 hackzall, the M12 Fuel Hackzall, M18 Fuel Sawzall, M18 Fuel One-Key Sawzall, & M18 Hackzall
I can't recommend the non-Fuel M12 after using the Fuel. Significant difference. There is a noticeable size difference (fuel being beefier), but still compact. I have been able to stall the non-Fuel, where the Fuel has kept going. I would never try to cut a 2x4 again with the non-F M12, thats for sure. Neither will replace a full size 18v sawzall. They have thin-kerf short blades for a reason, and the M12 hackzall is it, where as I don't hesitate putting any blade in the M12 Fuel. I love using mine with pruning blades.
If you need a 12v solution, go with the Fuel, you will just wish you made the jump up later on. You can get the bare tool w/ free 6ah battery for $139 i think @ HD.
The M18 hackzall is very similar to the M12. Slightly longer stroke, but a step below a full size. If I didn't have either saw, I would pick the one that I had batteries for already, if that's all it came down to. I would also consider what you will be cutting. If you will more often need 1 hand on the material, then I'd recommend the hackzall style.
Comparing any of those to the M18 Fuel sawzall is sort of a non-starter. Two different leagues. I've used my M18 Fuel to cut down a 15" Maple, when I didn't have access to a chainsaw. None of the other saws would have handled that, but we are also talking twice the size tool. The M18 can be held one handed, and once the blade is on the material it isn't so bad, but I think you get a better/faster cut with 2 hands on the saw.
In all of these cases, use an XC battery. Being a high-amp draw tool, you will notice an increase in power with the larger battery packs.
I have not own the regular M18 sawzall, or corded version. With all the deals involving the M18 Fuel model, I wouldn't have considered it, as I don't like buying old or lesser technology if something else is available. The One-Key is probably unneeded, but I do like having 4 different cutting profiles to choose from in an instant, and toggle the blade brake option.