What would be better, buying game mats or individual tiles for D&D?

I'd go for the tile sets you can buy as pdfs onlline. There are lots of them available, some of them for free, and they offer greater flexibility than a flip mat. You can print out as many of them as you want and arrange them however you like. Another option is a vinyl battlemat covered with a sheet of clear plastic. You can draw your original combat settings on it, and even slide dungeon tiles under the plastic so you can mark the combat area as desired.