Have you looked at the answers to this question?

https://www.researchgate.net/post/How-do-i-get-to-know-the-actual-chlorophyll-content-of-a-maize-plant-from-the-readings-from-a-SPAD-METER-What-is-also-the-standard-value

Thanks Mattew

But the question is which one is more reliable?

For me, I think the SPAD value is a better index of leaf greenness than leaf chl concentrations that are measured using laboratory techniques with human interfering of several steps!!

I just want to ask from scholars: do you this so?

I don "t think so

SPAD is at least preference as compared to labortary method