I've played PotC with the New Horizons mod extensively when I was helping with the mod 8 years ago, and 250 hours more recently. I've played stock COAS and done some modding on it, about 200 hours, but I don't have any experience with the various mods except to install and try them out. I've got over 1300 hours currently in TEHO, including 800 hours before I started modding it. I play sandbox in these games, and haven't done much of the main story line in any of them.
I've posted some comparisons on the Steam forums. Start reading here:
This vs City of abandoned ships? :: Sea Dogs: To Each His Own General Discussions [ENG]
...and follow my link there to the COAS comparison, and to my PotC comparison from a link in the COAS thread.
For a quick overview, TEHO and COAS are the same game with different quest content. TEHO has much more and it's much better than stock COAS. TEHO also has some improvements to the interface. I'd say that TEHO is the harder game because of the way the quests are written, but their quest writers were very good indeed. The PotC New Horizons mod is based on an older version of the game, but probably has the most added story content, and the added content is very good. I suspect much of the non-story content (ships, items, etc) added to COAS mods originally came from New Horizons, although there is a lot of new stuff there as well. New Horizons has had a long time to mature, and it shows.
Each of these games has a following, people who have it as their favorite. I'd say that New Horizons with its incredible wealth of content is probably the best game, while stock COAS is more modern and the mods add a lot, although you may need a powerful computer to run some of them. TEHO, while being more limited in content, and only recently becoming moddable, is the most modern and polished of the three. TEHO is the most stable, followed by COAS stock, then the others. New Horizons is generally stable enough, although something in one of the mods I wrote was causing almost constant crashes which I wasn't able to fix. I didn't play the COAS mods enough to experience any crashes, but advocates of those mods admit that TEHO doesn't crash as much.
While I prefer TEHO to the others, it is not without problems. There are some translation issues, and many of the quests almost require a walk through. You have to read dialogs very carefully because not all information is recorded in the quest log, and this is intentional. There is currently no free play start, and you will find your self playing several hours before you ever get your ship. After over a dozen games started, and knowing exactly what I have to do and in what order to do it, I can get my ship in just over an hour, while the first time took me 7 hours of play. Most players report that the story lines have considerable depth and are very interesting, but the game does not hold your hand and guide you through. Expect to reload from saved games often. The game had a number of things that annoyed me, but I was able to fix them in a mod.
Which game you choose is up to you, and you probably can't go wrong with any of them. A lot will depend on your preferred play style, and I'd say that New Horizons gives you the most flexibility.
Hook