This paper presents the results of a formal experiment to compare different interaction techniques across two types of immersive display: an immersive projection technology (IPT) and a head-mounted display (HMD). Our aim is to investigate the effectiveness of two widely used interaction metaphors, virtual hand and ray casting, on these two display technologies. Our motivation is that design and evaluation of interaction techniques for immersive egocentric display systems has been undertaken almost exclusively on HMDs. We argue that basing interaction for IPTs on techniques developed for other types of immersive systems is a flawed approach, as there are some categorical differences between the experience given by an IPT and an HMD. For example, an IPT user has a much wider field of view than an HMD user. We have chosen two types of interaction tasks to study: simple selection of objects both near to and at some distance from the user, and manipulation of objects involving a change of both position and orientation. As previous studies have found, we find that ray casting is preferable for selection and virtual hand is preferable for manipulation for a HMD. We show that this is also the case for the IPT. More interestingly, while we find performance on selection tasks is much better on the IPT, for manipulation tasks there is little difference between the two display technologies.