You MUST send it to the Service Center -- the DO does not accept the N-400 directly. If you send it to the DO, I suspect they'll send it back to you (or if you're lucky they'll forward it to the SC). That would only add delays.
The SC actually does most of the front-end processing of your case -- they process the fee payment, enter the information off of your N-400, send out the receipt notice, schedule and send out your fingerprint appt. notice, initiate the name check -- all before they transfer your file to the DO. Even your interview notice and oath ceremony notice / form will be mailed out by the SC (even though the DO is handling your case at that point).
