I have found that the park dresses are slightly nicer than the
Disney Store dresses though both are nice. DD tried on a Disney Store dress from last season and found it very itchy and uncomfortable. However, going to the store recently, I thought that the textures of the latest dresses were slightly softer and, if worn with a slip or undershirt should be fine. Another alternative that would give you a good price combined with comfort would be the princess nightgowns at the Disney Store. (25% off this weekend!)
DD wore park dresses bought used and new on eBay during our latest trip and, even when not wearing an undershirt, was completely comfortable. The quality of the park dresses is good, but not $70 good, in my opinion. As I said, DD's were three separate eBay finds that combined came to the same total as one in the park dress. DD wears them all of the time at home and they wash pretty well in a mesh bag on gentle cycle and hang dry. The older park dresses are the most durable of the bunch and look new with the exception of some lost glitter after over a dozen washes.
We also have a dress from an Etsy seller. It is the most beautiful and the most expensive. If I could afford it, I'd purchase all of DD's princess dresses through Etsy as they are both beautiful and original and I feel good about supporting talented crafters with small businesses.
Another option for a gorgeous quality princess dress is a company called Bijan Kids. The dresses run around $100 but they come up on a deal site called Zulily every few months for around $50. My mom bought one for DD and it's really stunning and slightly different from the park dresses. They just sold last week on Zulily, so you may want to ask when they'll be presented again.
Phew! It seems like I typed quite a bit there. I think I am a little too familiar with the ins and outs of princess dress purchase due to my daughter's princess obsession.