They should all net you about the same.
What all is done to it?
I've had a ZZP tune, Overkill, and an Intense. ZZP tune was meh. Saw up to 4* of knock on a 3.4 setup with only 14* of timing at WOT. Switched to an Overkill tune on the same setup, and no more knock. Overkill has a pretty nice 'quick star' feature. I was pretty impressed with it.
I'm currently on an Intense tune (totally different car) and couldn't be happier. Great customer support (they called me and talked with me for a bit about what was done to the car, what I wanted from it, and driving habits) Didn't have to do a security relearn (had to with the ZZP and overkill) and it didnt pop the code for a CASE relearn (though it's a good idea to get it done anyways)
From my experience
Intense >>Overkill>>>>>>ZZP
