Did you know this about the S curve? I didn't!

[Stroker]

My flow is Lum for various reasons. Ro's flow is Lum for his reasons.
If you dig Lightness in Lab, go for it.
Personal preference and all that jazz.

In the example you posted, the difference is negligable to me. That's fine.
Dramatic to you and perhaps others. That's fine, too.

I think the major difference between Lum and Light is largely gamma.
Gets into the hardware aspect of Cie xyz.
Theoretically, you should be able to get one to match the other with small % error with the mid-tone slider in Levels.

Whichever you prefer.