Obtaining feedback from time-constrained end-users is a major challenge in evaluating novel systems for specialised applications. The performance and feedback of engineers and surgeons was evaluated through an experiment where participants were asked to identify tumour locations within an anatomically realistic silicon liver model across three different conditions of an Augmented Reality (AR) prototype system (Baseline, Split AR and Full AR). Our findings show that engineers and surgeons share some similarities in their performance, feedback and behaviour, particularly when reliance on the AR system is high for both groups. However, engineers typically focus more on accuracy of the image alignment and are more accurate in their responses when supported by AR. Senior surgeons typically perform faster and use AR as supplementary information, while the performance of junior surgeons is more closely aligned to the performance of engineers. We conclude that engineers could be involved in preliminary evaluations of a surgical system or in evaluations of systems which are aimed at training junior surgeons, but that it is essential to involve surgeons in later evaluations, where ecological validity is a more important consideration.