Which of these Java conventions has higher priority?

It's not a matter of convention - it's a matter of what's logical state in the object.

If a variable is logically part of the state of an object, it should be declared as a field - and yes, conventionally put at the top of the file.

If it's only part of the local state of the method, then it would be mad to declare it as a field. This isn't in conflict with the convention of where you put fields - it's simply a matter of choosing whether it's a field or a local variable.

Within a method, I see a lot of novice Java programmers declaring all their variables at the top of the method - whereas a more useful habit in my view is to declare a variable as late as possible, and with as small a scope as possible.