How can you tell if you need to go heavier or lighter?
Really it takes trial and error to determine with a particular rifle. All we can do is talk in generalities and what works in our rifles. Odds are the H buffer would do fine in your AR but many midlengths have standard weight carbine buffers and do fine too.
In an otherwise properly functioning AR (no gas leaks, etc.) if your light powered ammo causes short stroking then you may need a lighter buffer. Like mjkeat said, it's best to go with the heaviest buffer that works reliably. This is critical in an auto/burst rifle to eliminate carrier bounce but not as important in our semiautos though.