Lawbringer (but just my opinion. Basically said it since you usually end up killing the targets anyway. ex: raiders)
These are basically the same thing, except that it is easier to find ears with the lawbringer seeing as raiders give you ears. If you are an evil character, you'd be better off going with Contract Killer.
Personally, for me, both are awesome perks, as control of your karma level is one of your greatest assets in Fallout: 3. There are many benefits of being able to switch between a good or evil character, access to Paradise Falls, or getting handouts from Megaton Settlers are specific to your current karma level. Also, remember that adjusting your karma level positively can also be done easily by donating to the church in Megaton, or Rivet City, or giving water beggars bottles of Purified Water. Negative Karma can be earned (if you dont want to kill essential people) by going into Moriarty's Saloon and accessing his terminal repeatedly, or stealing, until the desired amount of negative karma is achieved. Lawbringer and Contract killer are also beneficial in that they net you caps, and its easier to turn in ears/fingers for karma that having to do the aformentions steps to get your desired amount of bottlecaps. Personally, i always get both of these perks.
I never tried for both that often. It was pretty hilarious when I was getting both Regulators and Talon company depending on where I fast traveled after I went from good to bad quickly. Almost too easy to get slaves for Paradise as you just mezz the last Regulator in the bunch, like they are volunteering to wear the collar. The biggest problems are making sure they get to Paradise Falls alive, and taking time out of what I was doing to go get a replacement collar so I would have one when the next group showed up.