Both TVs provide standard built-in audio performance suitable for regular usage, though external speakers can enhance the experience. Both TVs come with smart platforms that support popular streaming apps and voice control features. Both TVs offer adequate brightness levels suitable for most indoor lighting conditions. TCL 43S451-CA and TCL 40S5400 differ in resolution (UHD vs 1920 x 1080), which can affect overall clarity and sharpness of the display. From a practical standpoint, both options provide a reliable viewing experience for most users. There is a difference in screen size (43 Inch (109.22 cm) vs 40 Inch (102 cm)), and a larger screen generally provides a more immersive viewing experience. Both TVs provide decent motion handling, making them suitable for everyday viewing including sports and action content. TCL 43S451-CA and TCL 40S5400 are two competitive smart TVs offering a mix of display performance, smart features, and overall value for different types of users.

Verdict: Choosing between TCL 43S451-CA and TCL 40S5400 depends on your usage, whether you prioritize display quality, smoother motion, or smart features.