You will need to carefully select the type of frame to match the style of your house.
In general, good detailing and sealing is critical to performance. Are the windows best to open outwards or inwards? That is, does the seal form a part of the opening section or does the opening section close against the seal? You also need to look and see how any rain that might penetrate round the edge of the window can escape. All of this requires close inspection of an actual sample; diagrams are helpful but don’t tell the whole story.
Something to bear in mind is that some materials expand and contract in relation to the atmosphere. When you are aiming to eliminate virtually all air leakage, quality detailing will be important to check.