I was not impressed with the brakes of the 2010 Jetta; they are spongy and braking response is unpredictable. I replaced the front rotors with cross drilled rotors and a composite brake pad. There is a noticable improvement! Additionally, I will replace the rubber brake lines with stainless steel lines and the sponginess will go away completely.
If you're experiencing spongy brakes on a traditional hydraulic braking system (not one that has electronic controls actuating hydraulic pistons)there is almost certainly air in the system. A power brake bleed should cure your problem at a relatively low price point. This is especially true if as I understand your post you're experiencing "unpredictable" response from the brakes, when an air bubble in the system settles in the caliper or piston it will react differently than it will when it is in the wheel line or master cylinder. BTW a brake bleed from a reputable brake shop shouldn't set you back too much money and has the added benefit of completely flushing your brake fluid for just a few buck more, IMHO well worth the money before you start swapping out expensive rotors, pads, and lines. That being said, braided brake lines are a nice upgrade and you've already ponied up for the rotors and pads and slapping them on will require a brake bleed anyhow so it might not be a significant cost for your case at this point.