I dive with Red Sail at least once almost every week for the several months I am on GC each year, plus I do several dives while there with one of the other fine operations as well (but not CTD) - so I can do a limited comparison.
First, most of the dive ops on GC are great, including Red Sail, and the dive experiences are very similar.
@boulderjohn, not sure when you dove with Red Sail, but that is not what I have experience over the 50 or so dives I've done with them. They do run large boats, but that means lots of space. I have gone with a few of the 6 and 8 pack dive ops, and when those boats are full there actually can be less room than on a Red Sail boat. The dives are deep (100'/50 minutes) followed by shallow (60'/60 min), Both dives are guided but if you have a buddy you can buddy up and do your thing on the second dive. This is pretty much the same protocol I have experienced with the other shops unless my party is the only one on the boat. Red Sail breaks the group into small dive groups each with a DM, and these groups are often smaller than I have experienced with other dive ops - but not a big deal if you are talking 5 vs 8, say. They also try to break them up by skill level, although you can go with friends if you like. Red Sail is very accommodating. I took a friend out for a discovery dive, and wanted to dive with him. They said they could not do that, as the DM has his full complement of four discovery divers and could not be responsible for me. So they voluntarily without my having to ask assigned me my own private DM (no additional charge), and the plan was to hook up with them at some point. I have taken friends on discovery dives with other ops, but I had to register as one of the discovery divers to go along. And BTW, that Red Sail dive was 76 minutes for me (we went in about 30 minutes before the discovery group).
In general I find that with most of the ops, including Red Sail, bottom time is not hard and fast. They all seem to do the 50/60 minute thing, but if you are first in the water like I try to be, I am always able to get though all my air with Red Sail and the others. At the end of each dive, Red Sail will buddy you up under the boat (if you don't already have a buddy) with someone who has similar air and you can pretty much stay down until you and your buddy are ready to surface. Again, not unlike the other dive ops I go with.
One last thing, I find Red Sail to be very professional. I have had minor equipment issues, scheduling issues, and the like with some of the smaller ops. Never with Red Sail. I do also like the smaller ops, though as some of them provide more valet-oriented services (although with any of them , all you have to do is ask) and can be more flexible regarding dive sites, especially if there is only a very small number of divers on board. Plus a couple of the smaller ops are the only ones doing 2-tank PM dives, which I prefer.
Again, i don't know much about CTD but you probably can't go wrong with either.
