I use them both. I'm goofy!
The stainless loads from the center out while the plastic loads from the outside into the center. I've got the Paterson system where I can develop multiple reels at the same time while the stainless can I have accommodates 1 reel at a time. Just what I've got.
They both work very well. The stainless can I can put into a tub of water to maintain temperature and I find the stainless transfers heat/cold quicker than plastic. I find the liquid pours in faster & drains faster with the Paterson tanks. They use an upside down funnel for the lid!
So it's the same horse just a different color!