Well, not sure anything I am going to say is going to be of any help at all to you since this is just my opinion, but I will tell you what I know.
When it comes to metal or plastic speculum's, I like the metal ones the best because I am old school, and while I do like them warmed up first, the metal ones just have a nice weight and feel to them that the plastic once don't offer. The plastic ones are too light, but the trade off on them is that they are not as cold as the metal ones can be. However, with that being said as well, the metal ones from my experience open and close much quieter than the plastic ones do. For example the plastic ones make that cracking almost scary creeky noise as they are being opened, not to mention they are very jerky and that for a lot of women, especially women who are getting them for the first time or who are hurting down there in that department, the clicks and ticks of a plastic one I'm guessing would be a little bit frightening to them. Another thing with the plastic ones, or at least the ones that I have seen used a "for lack of better explanation" use a ledge/step system to open the speculum up. So sort of like someone walking up a set of steps in tap shoes, there is nothing quiet or smooth about that. You hear the hard click and noise, then feel the little more stretch. The plastic ones from what I have seen or have used just seem so archaic. Yes, I know they are supposed to be a modern improvement, cheaper to toss them than to resterilize them in the end,but how much if any more comfortable or cost effective could they be?
When I go do my doc, I made sure he had metal speculums to use, and he does and he made a note in my chart accordingly, so he only uses the metals on me.