neither will give you "best results"..
It dpeneds on your situation..
2 ohms = more heat generated
2 ohms = more power (in most cases, some amps output the same amount of power at any ohm level
4 = "want to be" in car audio atmosphere's, (read that somewhere)
depending on if you amp is stable to 2ohm mono.. (if you are working in mono, IE: monoblock amps)