Now take this for what it is worth as I don't have a veneer press (yet) .... but I've researched it a bit and here's what I gather:
The Poly bags are much stronger, less prone to holes and durable. They stretch more, some are less prone to glue sticking to them, and they are more flexible. The vinyl are good too and usually a little less expensive (used to be a LOT cheaper, but not anymore).