I bought a used Diablo from a local seller so I have a little experience. The seller sold the tuner locked. He set it to factory HP option, which doesn't unlike it. I contacted Diablo and was told he needed to set it to Original Backup. Well, he set it to original backup and now my predator works. So if you set it to Original Backup, the tuner will be unlocked.
As far as SCT vs Diablo they both do about the same thing. One of the best tuners here in So. Cal prefers Diablo. Advantage of SCT is more Email tuners support them.