When using found wood, you have to have a good metal detector to check out every piece before you work it. You also have to use a wire brush to find and remove small stones that often get ground into the wood, particularly with pallets. Those little bits will wreck the blade of both jointer and planer, leaving a small ridge on the workpiece.
One of the important uses of a jointer is flattening pieces to glue up for a larger panel. I know bigger might be better, in my opinion, glueups of narrower pieces are less likely to warp, so I haven't needed more than a 6 inch jointer.
I can see needing something wider if you're working with rough lumber. Lots of good input here.