There are two reasons why you would want to use UX Write over Apple Pages. The first is that UX Write supports the .docx format better than Pages. Secondly, UX Write is the only word processing app I am aware of that does auto-correct with an external keyboard. Even after the iOS8 update Pages does not do this although many important fixes (including better Japanese input in zoomed mode) have been made.
These two reasons alone are enough for me to continue using UX Write in view that MS Word has not made it to the iPad here in Japan. And Microsoft’s Office Mobile, though not found directly in the App Store, is a poor replacement for MS Word as well as being quite unstable. But when most people and businesses still use MS Word for their word processing needs then I have no choice except to stick with anything which can do .docx. If being compatible with other people’s work is not an issue then my choices are open. Pages then would be a fair runner since it is a good app save the fact it doesn’t do auto-correct with an external keyboard. But when you need to exchange your work in a common format then the choices are narrow. I guess we’ll have to wait for iOS9 for that.
Having said all this UX Write is such a good word processing app in itself. It is fun to use if word processors can be fun at all. As someone who loves to write I do think they are fun. And UX Write is one of these apps that make it enjoyable.