Electronic tuners are used to fine-tune your vehicle's engine to perform the way you would like it to.  This can include increased horsepower, disabling Displacement On Demand (DoD) / Active Fuel Management (AFM), using different-sized fuel injectors, or using different types of gasoline.  

A tuner is also a gateway device that allows you to tune your vehicle to work with different modifications -- that is, once you have a tuner, you can further refine your engine to optimize power and fuel efficiency.  

DiabloSport® makes several portable, hand-held tuners that come with several tunes pre-installed, including tunes to increase power and disable DoD.  These tuners -- the DiabloSport Predator®, the DiabloSport inTune®, and the DiabloSport Trinity® -- can bump the horsepower of an otherwise stock LS4 by 10 to 15 horsepower.  

Unfortunately, the DiabloSport Predator does not work on 2007+ LS4s which have the newer E67 ECMs; they only work with the 2005 - 2006 E40 ECMs. So, if you have a 2007 or newer LS4, you can not control the Active Fuel Management (AFM) or Displacement on Demand with a DiabloSport Predator; you'll need to get a DiabloSport Trinity or a DiabloSport inTune.

The DiabloSport Trinity -- DiabloSport's most advanced tuner available -- and the DiabloSport inTune are able to tune your Transmission Control Module (TCM) while the DiabloSport Predator can not.  TCM support allows you to modify settings such as shift firmness, shift speed, and shift points.  

Both the DiabloSport Predator and the DiabloSport Trinity use the same tune format, so from that perspective there's no performance advantage to going with one over the other.  The inTune, however, uses a different tune format.

Moreover, an LS4 community member, 06MonteSS is able to put together custom tunes for the DiabloSport Predator, the DiabloSport inTune, and the DiabloSport Trinity that can extract even more power (often more than 20 horsepower) from an otherwise stock LS4 and can be even further refined to match your exact vehicle.  I've worked with him for several years now, and I have to say, he will go out of his way to build a custom tune that works great for you.  

I've used a DiabloSport Predator since 2007 and it works wonders.  I've disabled DoD and tuned out an error code as a result of modifying my exhaust, and the increased power is definitely noticeable.  The fuel efficiency I get with my DiabloSport Predator tuner / DiabLew Tune is significantly better than the stock mileage.  With my driving habits, the improved fuel efficiency saved me more than the cost of the tuner my first year with it.  What you get out of yours will likely vary, but it can, in all likelihood, save you some money at the pump. 

I had to drive my car for several weeks without a tune and I could instantly tell the difference.  Without the tune, the car felt sluggish and unresponsive.  I was really impressed at the difference (and sorely disappointed when I had to go back to the stock tune for a while).

That said, the DiabloSport inTune and the DiabloSport Trinity add a bunch of features that are definite improvements over the DiabloSport Predator.  Also, as the DiabloSport Predator is DiabloSport's oldest tuner for these cars, enhanced functionality will likely come to the newer DiabloSport Trinity and DiabloSport inTune devices.

Unlike DiabloSport's products, Overkill's product actually involves swapping the ECM from your car.  Fortunately, this is a very easy task.  However, you're left with the options of either sending your ECM (and / or TCM) to Overkill and having your unit reprogrammed (at a discount) or you can pay the core charge for a new ECM (and / or TCM) and have a new unit shipped to you.  If you have your parts reprogrammed, your car will be stationary until your parts return to you.  Overkill's web site states that their business is in Canada and that they ship twice a week; so, expect your car to be out of commission for several days.