I spent a couple hours the other day trying to learn the difference between the two. The primary is what I read online.
Leaf Spring with
axle:
Pros: Cheaper, Easier to work on, can replace parts, and large bumps/potholes are absorbed through wheel, spring,
axle, and other wheel.
Cons: Can break down easier, do not have as long warranty, creates a bumper ride (no dampening), louder, more attention required
Torsion Half Axles (not full bar across):
Pros: Last longer, warranty is longer, smoother ride, less attention required
Cons: More expensive, cannot replace just 1 part need to replace whole unit, major potholes to wheel does not disperse energy through other channels.
In reality, one needs to asses what suits them. For me, Torsion
axle it is, as I will use mine every other week or less. Using for bikes and toys it needs to be smooth.
There's much more to it if you spend the time, but that's the just of it all. Once you add the Axle and 2 leaf springs, its not a huge difference of price to 2 torsion axles.