Complex sentence: The main difference is that a saddle-type valve has its two ports in line with each other on the opposite sides of the valve, whereas the seat-type has the in/out ports located at a 90 degree angle from one another.
Split into simpler sentences: The main difference is that a saddle-type valve has its two ports in line with each other on the opposite sides of the valve. Whereas the seat-type has the in/out ports located at a 90 degree angle from one another.