Insignia NS-39DF310NA21 features a 39 Inch (99 cm) screen while Westinghouse WH40FX51 comes with 40 Inch (102 cm), which can make a noticeable difference depending on your room size and viewing distance. 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. Insignia NS-39DF310NA21 and Westinghouse WH40FX51 are well-equipped smart TVs that aim to provide a balance of picture quality, performance, and smart capabilities. From a practical standpoint, both options provide a reliable viewing experience for most users. Both TVs provide standard built-in audio performance suitable for regular usage, though external speakers can enhance the experience. Both TVs offer adequate brightness levels suitable for most indoor lighting conditions. Both TVs come with smart platforms that support popular streaming apps and voice control features. Both TVs provide decent motion handling, making them suitable for everyday viewing including sports and action content.

Verdict: Choosing between Insignia NS-39DF310NA21 and Westinghouse WH40FX51 depends on your usage, whether you prioritize display quality, smoother motion, or smart features.