My impression is that most use aluminium (pans, that is; I have never paid much attention to the spoons, but I would expect them to be of steel). For myself, I have used aluminium (plain and non-stick), stainless steel (ditto), iron (ditto) and ordinary steel pans (with nylon and/or wooden spatulae), and have not been able to detect any significant effect on the flavour. At the moment I am using an extremely cheap non-stick mini-wok made of iron, and the results I am achieving in that are indistinguishable from the effects I have achieved in implements made of other materials. The great advantage of the mini-wok is that, having high sides, it much reduces the splashing and spattering that is normally the bane of curry cookery.
** Phil.