Probably the video card has a more sophisticated scaler so you'd get the best results. But, that would make your video card work a tiny bit more. You could use the display's scaler to free up the video card from doing that tiny bit of extra work. Maybe try it both ways back-to-back and see what looks better to your eyes. If it looks the same, then perhaps use the display's scaler to help share the work between the video card and display.
However, using the video card's scaler can open up some new opportunities that may not be available on the display. For example, you may want to use letterboxing to change the aspect ratio (e.g., use a 16:10 display to output at 16:9), but your display keeps stretching the output to fill the screen, so use the video card scaler for that. However, I think most displays let you choose whether to stretch, so take a look around inside your display options.