Wider or longer focal length?
Generally, the wider the better as it affords you to be able to shoot faster, save faster, and have fewer images to process and retouch. However, the drawback is that you get less overall resolution in the end if you shoot 6 images (for a horizontal row for example) than if you shoot 12 shots for a horizontal row. Wide angle lenses generally have more chromatic aberration as well. That is not a huge problem, but something to be aware of. However, if you will be shooting Gigapixel panoramas, then you will want to go with increasingly longer focal length lenses.