I procrastinated for a long time before finally deciding to go the RoSPA route, in the end more by chance than objective decision making! As said above both teach based on the same book. I too prefer the idea of a three year re-test (mine is due this year) also the re-test is free provided you remain a member of RoSPA and additional tuition is always available for the cost of tea and fuel for the tutor. I've not done the IAM training but I understand it is very similar.