They both have advantages over the other. People who claim that one is clearly superior than the other don't know what they are talking about. A pad allows for more fluid movement (e.g circle movement and generally just more advantageous for dodging), but a keyboard is more precise (e.g micro tapping, especially if you have to re stream, like on Hourai Jewel or Honest Man's Death), but that doesn't mean that you can't do precise movement on a pad or clean circular movement on a keyboard. It's just a matter of practicing those things enough.
Personally, I use a keyboard. I tried using a pad two and a half years ago, but felt like adjusting to it would take too much time for it to be worth it, so I just stuck with keyboards and I don't think I'll be switching at this point.
Pads are great for platformers though w