In diagosing pancreatitis, which is more useful, MRI or CT? And are regular abodominal MRI and CT different from MRCP or HIDA scan?

Each has its place. CT is the standard study for demonstrating pancreatitis but usually it can be diagnosed by blood test and clinical exam. CT is usually reserved for looking for complications. It's faster & cheaper than MRI or HIDA. MRI & HIDA are used to look for causes of pancreatitis, like obstructed ducts. Most cases of pancreatitis require only simple ultrasound to rule out obstruction, and no other studies.