Whichever one keeps your computer cool enough and doesn't annoy you. With the Silent Series R2s, you can probably just put them at 12v and leave them there.
One thing to note though - some people have had issues with that integral controller because of the 7V setting. Basically, when they switch fan speeds with the PC running, it can cause the Blue Screen of Death (BSOD). It's not exactly a common event, but I would try to avoid switching speeds while it's running.