YSH is possibly our favorite WDW restaurant. The steaks are always cooked to your specifications. The room is lovely, the service is first rate. The lobster bisque is to die for - but not until you've had some of those pull-apart onion rolls slathered with roasted garlic and butter.
We really enjoyed Yachtsman Steakhouse, the food was delicious and we also had great service, and nice atmosphere. We only dined at The Brown Derby once, and really did not care for it. I just think that for what we paid, the meal was not worth it. Plus, I would not waste 2 credits on it, if you are on the dining plan.
We have not yet tried Yachtsman (maybe one day). We were not impressed with Brown Derby at all. Food was mediocre at best and service was horrible (we were there for 90 minutes and only five tables were occupied in the entire restaurant). We don't see it being worth two credits and have not gone back.