Both TVs offer adequate brightness levels suitable for most indoor lighting conditions. Insignia NS-39DF310NA21 and Westinghouse WH40FX51 are well-equipped smart TVs that aim to provide a balance of picture quality, performance, and smart capabilities. Insignia NS-39DF310NA21 and Westinghouse WH40FX51 differ in resolution (1280 x 720 vs 1920 x 1080), which can affect overall clarity and sharpness of the display. There is a difference in screen size (39 Inch (99 cm) vs 40 Inch (102 cm)), and a larger screen generally provides a more immersive viewing experience. In general, both TVs cater to modern entertainment needs, including streaming, gaming, and everyday viewing. Both TVs come with smart platforms that support popular streaming apps and voice control features. Both TVs provide standard built-in audio performance suitable for regular usage, though external speakers can enhance the experience. Both TVs provide decent motion handling, making them suitable for everyday viewing including sports and action content.

Verdict: Both TVs are strong contenders, and your final choice should depend on your preference for screen size, motion performance, and smart features.