I had this same problem deciding a few years ago. I chose to do GC because we were renting a jeep for the day in Coz and didnt want to have to stick around town for a scheduled tour time. The one in GC was nice, glad I did it once. My kids loved it. You can book this tour directly with Atlantis Subs online for cheaper than the ships tour cost, thats what I did. I cant say anything about the one in Coz, but in GC its very close to the tender area, just to the right. It was a well run tour, we were pleased.
