I've got one P-64 left now (my other one was stolen) and I've changed the springs out on both of them. Most people find they like the 18lb spring the best. Some particular guns have trouble with light hammer strikes and dropping magazines after the last round and those particular guns do better with the 19lb spring. My remaining P-64 was just the opposite. The 18lb spring still was too stiff, so I went to the 17lb spring and it worked just fine. My suggestion is to buy the 3 spring pack and see which one works for you. Also, you will probably find that as the gun loosens up over time, you may need to adjust up one lb from the one that originally worked.
As for the recoil spring, the 20lb seems to be the most popular.