I am in favor of designs where the cargo liner and seat backs are separate. When you clean the interior, it's easier to manage smaller segments than one single piece flopping around. You likely won't need to clean the seat backs as often as the cargo liner anyway, so being able to separate them is a plus. Also, if you ever need to replace a certain part that gets damaged, separate segments would enable you to replace only that mat.
Having said all this, I suggest you consider if you even need the seat backs at all. It's easy to get wrapped up with more and more gadgets that you may not really need. This is coming from me, someone who has full body PPF and frequently posts in the detailing/washing threads lol. I mention this to clarify I am fastidious with maintaining a clean and protected car, and I don't have seat backs. If you plan on hauling pets in the back or other dirty items, then maybe seat backs are a good idea for you. Otherwise, they're really unnecessary.
