I feel strongly that you should use an interface builder instead of hand-coding a GUI. As in the question mentioned it's a much cleaner separation and once something has to be edited it's much easier.
The Qt Designer got this feature to create a class out of a .ui file1), but I think that not using this feature is the best way, as this creates just more coded that shouldn't exist at all. The speed issue of creating the window from a .ui-file is negligible because the window has to be loaded only once.
This is PyQt, but something similar is possible in C++:
class SelectDateDialog(QDialog):
def __init__(self):
QDialog.__init__(self)
uic.loadUi("resources/SelectDate.ui", self)
Essentially this has the same effect as including all your UI-code into the __init__() method, but the UI is almost completely separated from the code.
1).ui files are XML files that describe a user interface
