Motorola serial number - which one?

I have just purchased a new Motorola Surfboard eXtreme SB6121.  I got this only because the Motorola SB5100 that I purchased from Comcast about 6-7 years ago is getting old.....but is still functional.

Now I have a simple provisioning question.  Please reply to this post only if you know the answer.

My question is as follows:

I know that I'll be asked for the serial number.  Which one of the two numbers printed on the box shall I provide?  The shorter "Customer S/N" or the longer number identified only as "S/N"?

Thanks in advance for any definitive help that you may provide.