Though I've made several visits to Spain, I haven't yet seen either Granada or Tangiers, but I think I've still seen enough to warn you against curt, blunt blow-offs like "the Tijuana of Morocco," or "it's a dump," which appear above. In my years of travel I've basically "specialized" in places that hold some promise of being historically or culturally interesting, but which are well off the beaten tourist trail, and which lack the Fabulous and Famous Visual Spectacles (like the Alhambra) that draw in the hordes. I've had great trips to cities and towns that more conventional tourists would (and sometimes did) dismiss as "not worth it."
Now if you happen to be a conventional tourist, dedicated to the Fabulous and Famous Visual Spectacles -- and it's a perfectly legitimate reason to travel -- then by all means choose Granada. All I'm saying is that when you do decide, base your decision on comments more thoughtful than the curt "it's a dump" type.
(Now if it were the universal consensus, from all quarters, that Tangiers is a "dump," that might be different, but as you observed yourself, it's not the consensus.)