Liquid for dry, normal or mature skin and for colour that reads as skin; powder for oily skin and for control. A liquid is pigment in a base that melts into foundation, so the edge disappears and the colour looks as though it came from underneath — and that same property is what makes it grab if you are slow. A powder sits on top, which is why it builds in thin layers and can still be moved with a clean brush afterwards. If your skin is oily and you set your base, powder stays and liquid slides. If your skin is dry, powder finds every flake and liquid does not.
This is not a quality question
Both sit at 4.5 and above with thousands of ratings behind them, and neither is the better product. They are different tools for different skin. When someone decides blusher does not suit her, she has almost always been handed the wrong one of these two rather than the wrong shade.
If your face is both at once
Combination skin is the normal case, not a special one. The usual answer is the liquid where the skin is dry and a little powder pressed over the part that goes shiny — one product doing the colour and the other doing the staying, which is not the same as choosing between them.
