We went on a tour to Appleton from Negril a few years ago with Reggae Tours and they were great. Private vehicle, just us two, Nice AC, cool driver/guide, everything inlcuded. I was going to use Reggae Tours to go to Dunn's river this June but I see that CSS has tours every day included! How are those CSS tours? crowded? is the entrance to Dunn's included? etc
CSS offers this excursion free of charge... it's one of our favorite excursions and so easy. It goes out at 9am pretty much every morning, just a very quick (10 min) trip on a comfortable a/c bus to Dunns River Falls. They take great care of you from pick up to drop off, all you need to do is wear your swimsuit and water shoes or very secure/ gripping sandals, take a waterproof camera, and take about $20 in $5 bills for tip money and more if you want to shop(not recommended, the vendors are too aggressive, but you just walk through the short exit and don't stop and you'll be fine). The resort will give you a bottled water and a towel for you to use after the excursion. The tip money we normally take is $5 for the shuttle driver, $5 each for the guides (two to three guides for each group), and then $15 if you want the picture they take of you (just check it out, 4 out of 5 times we've loved the picture and bought it). We always go on Sundays when the cruise ships are NOT in port. However we did go one time when the cruise ships were there, it was busy, but still a blast, you are always just in your group from CSS. You are back at the resort before lunch time.
If you want to do a trip to Dunns, I would use the trip from Couples. Why pay for it again when you already have. The bus trip and entry fee are all included, and it is the same if you go with Couples or if you go by yourself. We went the first year we visited CSS, and enjoyed it. The climb was fun, and is the same regardless of how you go. I would say save you money for some other trip if you want, and take the one offered by Couples.
There is no cost to do the tour. The only thing I would suggest is to look and see what days Cruise ships will be in port at montego bay and ocho rios, and try to go on the opposite days. There will still be people there, but it definitely did not feel crowded by any means when we went. Also, because you go in a line, whether you go with CSS, or go on your own, they are going to put you into a group of about 20-30 people, so you all hold hands and walk up the falls together in a line.