I bought mine because I liked the option to change the length. I am a big guy and the longest setting and middle setting work great for me. It saves a little weight I am sure too.
Lets all be honest to - a lot of the reason everyone gets it is because exactly like you said: it is how a Benelli M4 SHOULD look and it just looks cool.
I find benefit from the c-stocks ability to shorten the gun for storage, the M-4 is not a small gun and would not fit in the case I use without it. I am eagerly awaiting the rx-arms 6 position recoil tube. I can shoot mine presently from either the middle or third notch but neither feel quite right. I'm confident the extra positions will rectify that.
Sounds like your dealer is willing to work with you, offering you another gun as a exchange. Perhaps request a photo of the other gun, then make your decision.
The Nova is a very solid gun, it will last you a lifetime, super durable. I have worked on well over 1100 Benelli's, only three of them have been Nova's! 10 year Warranty...
I've owned a number of the C-stocks over the years, no longer have one, it does nothing for me. The only benefit I can possibly see is shorter stock position when wearing body armor.