Overall, both models are capable choices in their segment, offering a balance between performance, features, and usability. 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. TCL 43S451-CA and TCL 40S5400 differ in resolution (UHD vs 1920 x 1080), which can affect overall clarity and sharpness of the display. Both TVs provide decent motion handling, making them suitable for everyday viewing including sports and action content. TCL 43S451-CA features a 43 Inch (109.22 cm) screen while TCL 40S5400 comes with 40 Inch (102 cm), which can make a noticeable difference depending on your room size and viewing distance. TCL 43S451-CA and TCL 40S5400 are well-equipped smart TVs that aim to provide a balance of picture quality, performance, and smart capabilities. Both TVs offer adequate brightness levels suitable for most indoor lighting conditions.

Verdict: Both models offer good value, and the better option depends on your specific requirements and viewing preferences.