Depends on what you like to do. Personally, I would get the external HD, and hope that the laptop has either USB 3.0 or eSATA for faster transfers. Blu-ray would require quite a few discs, unless you get up into triple- or quadruple-layer. An external HDD can also be used on multiple drives
Before wiping your drive, please take the time to verify your backup. It would be a real pain if the backup could not be read.