If you have a mostly stock car nearly all of the hp gains from a tuner are the result of increased timing for 91-93 octane. If you run 87 octane there will be very little to no gains. If you don't want to run higher octane then there is no point on buying a tuner (unless you are moderately modified). Running 93 octane on the stock tune is a 100% waste of money and will result with slightly less hp and worse mpg.
i prefer the diablo myself. doc is probably going to sell his x-cal3 off his mach because we had better outcomes with the diablo. you can run 91on 93 tune without any issues. i can run 89 easily on my car, with my aggressive tune in it.
my name is mike:wave, but everyone will probably say :gives. if dyno tuning, use what they tune with. small tuning, either is fine, the sct is a little nicer i think, a lot of tuning especially with timing then i say diablo. you can pm doc8224 and get more info from his point of veiw of his x-3.
I have the diablo on my Mach, and have nothing but good things to say about them. I only run 93 octane. With the quality of fuel these days I might start buying race fuel. I don't kow if it's bad by you guys, but they water down the fuel by us.