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.
|Fangorn Forest by cand1d|
|Yosemite Falls with Moonbow by Jonathan Shapiro|
from Best Landscape of the Week 4