Get the Q90a. Its not much different from Q90b. The 50 inch version of Qn90a costs like 85k and with offers the price comes down to 76k only.
I feel the lower prices are due to localised production of Samsung. Sony TVs are very good but carry additional markup in India. Compared to US or EU Sony is much more expensive in India. X90 is a tier below Q90. Though x90j is available for 104k at shop at sc with cash back.