I use Jobo (plastic) drums and reels, which are particularly useful when you develop colour. The bleach/fix component releases gas and hence pressure and the Jobo lids have room for expansion build into the lid. Systems without expansion would most likely leak in that case. Also, plastic gives better insulation and thus keeps the process temperature more stable. I have not seen metal systems with that feature, but that does not mean they are not out there.. Apart from this, its all a matter of preference and what feels right to you.