Should you visit Milford Sound from Queenstown or Te Anau?
Te Anau usually gives Milford Sound a better rhythm; Queenstown is easier if you need tour infrastructure. The Queenstown day trip can work, but it is a very long day, not a casual scenic errand.
Answer
Queenstown is the convenient answer because it has the airport, tours, hotels and the sense that everything important starts there. Milford Sound is the exception where convenience can become punishment. The return day is long, and that matters if you are driving, travelling with children or hoping to enjoy the road rather than simply endure it. Te Anau is not as dramatic a base, but it puts you closer to Fiordland and lets the day breathe. Queenstown still works for travellers who want an organised coach trip or do not want to change hotels. Just do not pretend the two options are equal in effort.