Spike and Buffy may have actually been in love, but he hurt her and she couldn't do anything about it. So that's a reason why Willow and Tara are better because they didn't hurt each other like that, and they really loved each other and didn't just fuck each other because they felt lonely or out of place in the world. Another reason is because when Tara died in Willow's arms, Willow went insane and tried to end the world. When Spike died Buffy didn't cry or really care much. Not to mention Willow and Tara's relationship was more caring and sweat than Spike and Buffy's.