I use lojack, an alarm and a dsp5 theft tune that is hidden.
Onstar is crap for theft. A cop told me when my grandpas truck was stolen that they pull the fuse right away or rip the antenna off.
He also said a club was best bet, most thieves dont carry tools and if they see something like that they will just pass it up.
If you have good lojack service around you its prolly the best for recovery. or a hidden gps unit. I have installed a few gps units and they are awesome for other things too. Also I use a dummy gps untit, its easy to find under dash and they think they have disabled the gps.
Look up goldstar gps, they have cheap units that are awesome. Most rental car companies use them.
You can never have too much these days.