Ok, when I was in China, I personally tried the same cafe latte without sugar, from both starbucks, and luckin coffee. And just to be fair, I tried them at various outlets just to remove as much bias as possible.
I always stuck to the same time of the day to get the drink, at night, because I need a level of caffeine to get to sleep.
Overall, I find luckin cafe latte smoother to the taste compared to starbucks, which feels slightly more coarse.
I attribute it to the beans, and brewing method.
You ask me, they are both equally fine, really just boils down to preference.
There is no best, you determine that yourself.
Price wise, Luckin was 1/2 to 3/4 the pricing of starbucks, and luckin usually had discounts. Starbucks nada.