Think about what you're buying it for. Think of a line with foldability at one end and performance at the other and decide where on that line your priorities lie. So for instance, if you're a regular train commuter and are sick to death of trying to cram your 700c wheeled hybrid onto the train at rush hour, something that folds small and is suitable for riding in normal clothes over shortish distances is what you're after.
If, on the other hand, you're an ultra fit roadie after a bike that you can pack into a small case and take with you on flights, then you can sacrifice quick folding in favour of a more efficient ride. There are bikes out there which manage to do both of these things really well. As mentioned before, there are also good quality folding mountain bikes out there that would serve you well on the trails, but fold in half quickly, so you can pop them in the boot.