The scans of 12x and 8x burns you posted are both fine.
The quality of the 8x burn is better in my opinion, and from the scan I can see that the result would probably be even better at 6x.
How can I see this?
The NEC 3550 burns the disc in zones, starting at 4x, then 6x and finally 8x. This is also known as Z-CLV. Of the three zones, the 6x zone has the lowest PIE/PIF.
Whether you want to burn with good quality at 12x, better quality at 8x or even better quality at 8x is entirely up to you!