Frankfurt or Cologne for a trip to Dortmund?

Hi guys, I'm from the UK and could well be visiting Dortmund soon as it's looking likely my team mighy play them (Aston Villa).

I've heard Dortmund is not a great city to visit, so I think I'll save it for just the football. Would you recommend Frankfurt or Cologne to stay? I'm travelling with my wife (she chills while I go to the match). We would need to travel by train from either city to Dortmund.

Thanks in advance!