How each one actually works
Casement windows are side-hinged, like a door — they swing outward (or occasionally inward) on hinges along one edge. Sliding windows have sashes that move horizontally along a track, with no outward swing at all.
Ventilation
A casement window opens to its full width, which typically means more usable ventilation area for the same opening size compared to a sliding window, where roughly half the window is always covered by the other sliding panel. If maximizing airflow through a specific opening matters, casement generally has the edge.
Space around the window
Because casement windows swing outward, they need clear space outside the opening — worth checking for balconies, walkways, or anywhere something might be in the way when the window is open. Sliding windows need no such clearance, which is part of why they're common on balconies and tight walkways.
Seal quality
A casement window's side-hinged design lets it press firmly shut against its seals when closed and locked, similar to a door. Sliding windows, by their sliding mechanism, generally have a slightly less airtight seal by design — a small but real factor in insulation and noise reduction if that's a priority for the specific room.
Glass area and views
Sliding windows typically have a larger uninterrupted glass area per given frame size, since there's no need for the swing clearance a casement window's opening sash requires. For a room where the view or natural light is the priority, this is a genuine advantage.
A simple way to decide
Casement tends to suit rooms where maximum ventilation matters and there's clear space outside the window. Sliding tends to suit balconies, walkways, and situations where space or a wider glass view matters more than maximum airflow. Neither is a strictly better choice — it depends on the specific opening and what you actually need from it.