While I agree that it's not difficult, for a first time tank owner the whole concept of plumbing can be a bit overwhelming, especially when adding a manifold and anything other than a simple drain and return.
The plumbing that Neptune does is top notch. It's definitely worth paying a premium because they not only plumb it correctly, but they'll also make it look very presentable--clean joints, proper and even spacing between pipes, and the addition of unions where needed. One nice touch that Robert does is that he adds 45-degree adapters after the unions so that the pipe sits closer to the tank. It's not needed but definitely adds to the clean look of the plumbing.
Also, IIRC if you buy a complete set up from Neptune, they will give you a discount card good for future purchases. I don't remember the minimum size, but a 120 will definitely count.