[reboot]

Depends. If you do a lot of data transfer between the two, then put it on the second channel. If it's just storage, and not used all that much, slaving it will do. There's not a whole lot of difference in speed, but maybe just enough.
If it's an older slower drive, then put it on the second channel.
If it's as fast or faster than your boot drive, slave it.
You don't want to put a fast drive on the same channel as an optical device, it will just slow it down.