Keeper League - Keep Gronk or Dalvin Cook?

I have a chance to snag Dalvin Cook on waivers. I'd have to use one of my waiver claims (only 20 adds/drops per season) and stash him on my bench the rest of the season. At the end of the year I'd have to choose between keeping him or Gronk. If Gronk is the better option, there's no real point of using a claim on him. What are your thoughts? My other RBs are trash.