The differences between Spanish and Greek olive oils are not easily noticeable as these are usually mixed and sold in the same bottle. While the product maybe labeled as Spanish, it is actually a combination of oils from difference countries, including Spain, Greece, and Italy; only the blending and packaging is done in Spain. This is due to the fact that the per capita consumption of olive oil in Spain far exceeds what the country can produce. To meet the international demand for this oil, the country imports olive oil from Italy and Greece.
Actually there are more countries producing olive oil: Spain, Italy, Greece, Syria, Morocco, Turkey, Tunisia, Portugal, Algeria and others in the world. So I'd rather say every type of it has its taste and every is better for a specific meal.