I like to use Prey and have no complaints about it so far. However, our carrier also offers Mobile Recovery, which does a lot of the same things (GPS locations, etc). Is there any harm in having both of these apps loaded and running (other than it being somewhat redundant and eating up a little extra battery)?
Other than being redundant and using more resources (you would have two separate services running in the background all the time), you shouldn't have any problems running both at the same time like any other application. I would see it as not much different than running Locale and Tasker at the same time. They might do very similar things, but they aren't exactly the same.