I have slept on it, and I'm back at my computer where I can type out a more nuanced response. Thank you all for the feedback and food for thought! I'm sure some of you are thinking, "Just quit with the angst and buy or don't buy the dress, already" and I'm feeling a little like that too. It's just a dress!
However, this is a good opportunity for me to really think through an individual purchase, and I want to buy with both heart and care. Plus, I love hearing your thoughts on occasions, styling, and eco/social impacts.
So! Rayon vs. silk. As JenniNZ pointed out, thinking through the implications of various fibers can be paralyzing. I generally prefer natural fibers (silk, wool, organic cotton, linen) but I'm not opposed to rayon/viscose--in moderation, as I'm aware of the processing required to turn it into a usable fiber. The big thing for me with rayon is long-term durability. I'm not easily willing to pay as much for it as I am for silk. Does higher-end rayon hold up better than the stuff you find at H&M? I'd hope so.
Fashiontern, you asked about quality. I have only owned one Sundance piece, but I've tried on many, and I generally find the quality to be a bit above Anthropologie on average, and definitely more consistent. They do sell floaty, boho pieces, though, so you have to be more careful with them than, say, a flannel from Duluth Trading Co.
I've been thinking about occasions. Here's what I've come up with:
Shakespeare Festival - one of my favorite parts of the summer
DCPA events - We've occasionally gone to shows at the Denver Center for Performing Arts in the past, but we've vowed to increase our attendance in the coming year, so "dinner and a play" should become a more regular event.
art openings - When I had my work in an art show last year, I had to scramble a bit to find an outfit, though I was able to construct something out of my closet. Nothing on the schedule, but this could certainly happen again.
awards ceremonies - Son is a junior in HS this year, and I feel like these events start popping up for upperclassmen
Denver Botanic Gardens - We do events at the Denver Botanic Gardens every now and then. I think this dress would work for a garden stroll. I also do a craft show at the Gardens in early November, where the first night is a members-only evening event. I could definitely change into a dress like this after setup.
Even after moving my NAS trench coat purchase to spring/summer, I still have a slot open in my summer five. I just didn't buy many statements this season. Right now, my inclination is to wait and see if there's a discount offered this weekend, and then order and do some styling out of my closet.