Battery testing came up recently on a boater forum. Testers there were all electronic (not carbon pile) from $100 and WAY on up.
In HF, the electronic has the higher ratings.
I think carbon pile can be misused.
That said, the wording of the carbon pile ratings seemed stronger.
So for me, if buying one today: Carbon Pile.
With a warning: NOT down in the bilge of a boat, or where flammable fumes can accumulate. I don't think carbon pile is as "Spark Safe" as electronic.
I have no such tester. Given the choices here, I'd get Carbon Pile but I'd lift the boat batteries out before testing.