ORIGINAL: idunno
Good choice. Wait for a clear shot and don't depend on the multiple pellets in the buckshot to be more reliable to get through the brush. Hogs are extremely tough and I read somewhere that the 20 ga saboted slugs actually out perform 45-70 in equal bullet weights with less recoil.
For standard loads and cowboy loads a slug gun with the modern high power sabot loads can equal and possibly out perform a 45-70 on paper. However there are some newer high powered factory loads for the 45-70 and many hand load for it with loads that a slug gun can't come close to. I have had this argument before and lost. I like my crow with A-1 by the way.
But the average 45-70 load you see on the shelf you can get pretty close to it with a modern slug load, if your gun shoots them well. At least on paper, I can't say anything about real world performance since I don't own one and can't use a centerfire in my state anyway. As far as less recoil that probably depends on the rifle you are using. Perceived recoil is a pretty subjective thing.
Paul