I have been taught through my experience that if you're going to implement business systems to maximize efficiency and cost-savings, PLM comes before ERP. It makes sense to me because ERP deals with financials, and you really only get to that point if you are able to successfully build a prototype, and start moving toward production (and a PLM system helps you from the getgo, when you're first creating parts lists.)
But what do you think? Does anyone have experience implementing one before the other? What has worked, what hasnt?