This topic came up in the first 2001 paper, as Question 13: "The nurse notes a marked difference between blood pressure recorded via an arterial line in one arm and non-invasive pressure recorded from the other arm. What may be causing this difference? Which reading will you use to guide management?"
Which measurement would you choose? This is a judgement call.
One might wish to exclude all device-related problems before making a decision. Ultimately, one may wish to measure the NIBP manually on the same arm as the arterial line, noting the cuff pressure at the point at which the arterial trace goes flat.
The college, in their answer to Question 15 from the first paper of 2001, suggest that "if there is doubt ... a more proximal recording (eg femoral catheter or long brachial catheter or implantable transducer) may be necessary". They also remark that in a patient with severe peripheral vascular disease the higher pressure should be trusted.
Crul, J. F. "Measurement of arterial pressure." Acta Anaesthesiologica Scandinavica 6.s11 (1962): 135-169.