Both are brown or golden tequilas, but reposado ("rested") is aged very briefly in oak and añejo ("aged") can sit in flavourful oak barrels for up to four years, or even longer. Añejo tequilas have more brown-spirit flavours of cinnamon, vanilla and caramel, while reposados simply smooth out some of the pepperiness of white (blanco) tequila. Any of the three types is great in a Margarita - feel free to experiment!