Which test is better for diagnosing pancreas insufficiency: stool panc elastase or serum lipase/amylase? Would either diagnose pancreatic cancer?

Both together. Neither of the tests alone tells much. A combination of tests and exams will lead to diagnosis. Pancreatic cancer is not able to be ruled out by any test or imaging study. It can only be absolutely proven by some form of biopsy. A ct guided needle or egd with brushing a may be the only way to find it.