Both TVs come with smart platforms that support popular streaming apps and voice control features. Insignia NS-39DF310NA21 features a 39 Inch (99 cm) screen while Insignia NS-40D510NA21 comes with 40 Inch (102 cm), which can make a noticeable difference depending on your room size and viewing distance. Insignia NS-39DF310NA21 and Insignia NS-40D510NA21 differ in resolution (1280 x 720 vs 1920 x 1080), which can affect overall clarity and sharpness of the display. 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. In general, both TVs cater to modern entertainment needs, including streaming, gaming, and everyday viewing. If you're comparing Insignia NS-39DF310NA21 and Insignia NS-40D510NA21, both models come with modern features and are designed to deliver a solid viewing experience across different use cases. Both TVs offer adequate brightness levels suitable for most indoor lighting conditions.

Verdict: Choosing between Insignia NS-39DF310NA21 and Insignia NS-40D510NA21 depends on your usage, whether you prioritize display quality, smoother motion, or smart features.