It really depends on your budget. Also, it really doesn't matter if you get a 2, 4, or 6 channel LOC. If all you want is subs, a 2 channel will work. But, if the one you get has more, you can just not use the other channels.
If all you want is a sub out, just get one like this one. http://www.sonicelectronix.com/item_5285_PAC+TRUNK-LOC.html The thing that sets this one apart from the rest is a remote output wire to turn on the amp. It saves you from having to have to find a switched wire to use for the remote.
If you're planning on adding more to the system later, I would recommend this model. http://www.sonicelectronix.com/item_8168_AudioControl+LC6i+-WHITE+LC6-.html Pretty much the same idea as the other model. But, it's has 6 output channels with internal summing, level control and noise isolation.