My holders are anodized aluminium and acylic plastic. The alumimum will stain, but along with the acylic inserts they have remained robust for many hundreds of plates. Two tips: after draining your sensitized plate, use a paper towel to really absorb any silver bath droplets on the back of the plate, allow it to really drain well (at least 30 seconds, 60 seconds is better if the air is not too dry); and be sure to thoroughly wash out your plate holder at the end of the day: holder, darkslide, inserts all need to be throughly cleaned to avoid contamination and damage from the silver bath's solution. Yes, sounds pretty anal -- but the reward is repeatably flawless plates (ask Kerik - he's seen 'em)
NWG