Spun are probably 1/2 the weight and less likely to crack if mishandled/dropped, but may bend/dent but will survive panel beating back into shape.
If you can get them in Cres steel, then they won't rust at the slightest exposure to moisture like cast iron will.
Some have the lid that just sits on top with a lip to hold the coals, but the Bedourie oven sits over the top and makes it less likely to get ash in your dinner. You don't have the lip to hold the coals, but not sure if its much of an issue. When not being used as an oven you have a high sided plate and a large bowl and you can stick the bowl in the lid and pack stuff inside easier for transport.
Cast iron, being heavier, does resist temp changes so helps even out the heating and cooling of the oven, so is cooking temp is a bit more even. Not that it may make much difference. Packing they have a lot of wasted space.
They are also cheap and if looked after last forever, but there is that weight penalty.
I have a few cast iron camp ovens and stick with them because I can't justify replacing them at this stage.