You are right. These are very similar in construction and performance. Like other products, the main difference is in the reputation and reliability of the manufacturer.  However, because there are different manufacturers, membranes do come in several end type configurations depending on the manufacturer.  When ordering vessels, you will need to specify what brand of membrane you plan on using so the appropriate end adapters can be built in.  When replacing membranes with a different brand, you should check the membrane specifications to make sure the end configuration is compatible for direct replacement.