Define "low light performance"
In theory a combination of both - essentially how well it performs when you take an image in low lighting conditions.
In reality it will be biased towards the image quality, as it is the end result and a lot easier to quantify later on by comparing images. AF performance is much harder to nail down in a tangible form beyond getting doing an A/B between them.
|Meet at HKCC Rain or Shine by pen araneae|
from Your City - Rain Down on Me
|Pregnancy Is Beautiful by sye46|
from Women's World
|Criss-Cross Traffic by PEB|
from Best Use of Motion
|Picking a Star in the Milky Way by vbuhay|
from My Best Photo of the Week