Easy answer is always SCT, because there are far more local shops that use SCT software. I bought my car with DiabloSport tuning installed, and it ran like poop. I switched to SCT and had a local shop dyno tune it. The car runs fantastic now!
The other reason I like SCT is because of DiabloSport's lousy customer service. I have my son's DiabloSport Predator tuner which is locked. I called DS to find out what it would cost to unlock it. The smartass there said, " We don't unlock Ford tuners at any price, for any reason." I asked why they will unlock Chevy tuners for $150, and he replied, "You should have bought a Chevy!"
2003 Mach 1