I have only seen Dames At Sea, so I can't help you with The Color Purple. Dames At Sea is a very lighthearted show with some great dancing. There isn't much depth and it's borderline camp, but it's a fun night in the theater! Obviously The Color Purple is the stark opposite...so it just depends on what you want to watch!
For Dames rush, I think I showed up at 9:45 and I was the third person in line. Their rush hasn't been very busy at all, so I would plan on getting there right before 10.