[demick]

Yes, the underdrive power steering pulley is larger so that it will turn slower, and will only affect the power steering pump.

The crank underdrive pulley is smaller, and will turn all accessories slower (alternator, power steering pump, A/C compressor, water pump). This is why you get a power boost when you do the crank UDP, because you are reducing the losses of all of the accessories.

If you change both pulleys, you probably risk turning the power steering pump too slowly (but I don't know for sure).

Demick