Seriously though....Different hand helds have different pors and cons. The MAIN point you should look into..is what your tuner (or tuner of choice) uses.
Pretty much what he said. I personally would go with SCT because that's what most tuners use. However - if your local tuner guy uses Diablosport - well then it's a no brainer. I personally wouldn't go with anything other than those two.
i agree with RodeoFlyer 100%...most seem to use SCT tuners nowdays but if your local tuner uses diablosport by all means go with whatever will work best for you.
on a side note my brother just bought a SCT livewire and he seems to be in love with it...and it can tune any Late model ford since 1998 i belive. Hes also a Mercedes-Benz mechanic so he probly can get more fun out of it then most...but it sounds tempting even at the 500 dollar pricetag or whatever it is.
There are plenty of upstanding SCT dealers/tuners that can take care of your needs.
If the car is close to stock (mufflers/intake/gears)...then you may want to consider the SCT Livewire. It comes with preloads that will take care of your needs for now; but maintains the ability for custom tuning at a later date if you get a supercharger or other significant mods.