Effectively they are the same although with the zapper, the DC is literally pulsed. There is a bit of disagreement as to which is effective.
I do believe that with the DC method, they are using it to 'cook' the tumor and this apparently will also damage neighboring tissues. This does not appear to happen with the zapper. The cancerous cells are affected but surrounding tissue does not appear to be damaged.
The other difference is that with the DC tumor ablation methods, they place a bare wire electrode into the center of the tumor. This direct attack on the tumor is more efficient and if it were to be used with a zapper, it might improve the zapping effectiveness.