The first looks like a clamp and it sits on top of the spring and jaw like fingers compress the spring. The unit compresses well, but the down side is that there isn't a lot of room at the top for big fingers to remove and reinstall the valve locks.

The other type is a 'cat's paw.' This crow-bar-like device pivots on the rocker arm stud and the foot compresses the valve retainer. This works ok unless you have stiff valvesprings, which will make it feel like you're arm wrestling a gorilla. You also have to be careful you don't cock it and scuff the valve stem.

If I have to change valvespring with the head on the car, I prefer the first type. It is more secure, you just have to fight with the limited space for your fingers. Using a magnet on the locks really helps.

Did you know you can do this without compressed air? Just take cotton {soft} rope, put the cylinder at BDC, feed about three feet of the rope through the spark plug hole, then turn the motor by hand until the rope is compressed and holding the valves in place.