I'd do the course preview. I have my first triathlon coming up in a couple of weeks and in addition to regular training I did a super-sprint triathlon with a pool swim back in April and a course preview swim and run last week. That preview was in the lake where the triathlon will be but not the same course set up. Of the two, I think the course preview was much more helpful for preparing me for what race day might be like. Unlike you, though, I haven't done any open water swims as part of my training. But the crowds at the swim preview were a MUCH different animal than the crowds with the pool swim during the race in April. In April we started ten seconds apart so it was easier to manage, and because the swim was so short I never had to deal with more than a few people in the water around me at any time. The mass of churning swimmers at the OWS practice was very, very different and much more difficult to manage. I don't know what kind of open water swimming you've been doing, but practicing it with the buoys and turns and boats of a race course might also need some getting used to if that isn't part of the practicing you've been doing.
If you're already used to the adrenaline rush of competition you don't need any more experience with that, and all that's left is practicing transitions. You can easily do that on your own, and you can also set up the course preview so that you have your car as your transition area and practice it there. You won't be tripping over so many people, but it'll be a fair approximation.
Good luck!
Sarah