Is Willow more powerful than Tara?
Canon suggests this view strongly, but it doesn't prove that this is the case. While it's true that Willow casts more spells than Tara, there are a number of reasons for that ranging from Willow being more involved with the slaying to their different views on when to use magic with Tara being more reluctant to use her magical abilities. There's also the simple fact that Willow has more screen time.
When Tara expresses her fear of Willow's growing power during their argument in Tough Love, it seems clear that Tara thinks that Willow is the more powerful witch, though with Tara's lack of confidence in herself until season 6, I don't think that we can accept her self-evaluation here as an objective truth. We've also seen Willow need Tara's help in casting her most powerful spell up this point--the teleportation of Glory.
Willow does become more powerful than we have ever seen her before at the end of this episode after she reads the Darkest Magicks book, casting more spells in general as well as using spells that are more powerful than most that we have seen in earlier episodes. But does this mean that Willow is more powerful than Tara? This leads me to my next question.
What does more powerful mean?
Willow's reading Darkest Magicks increases her knowledge of spells useful for the Scoobies dramatically, allowing her to use her potential in ways essential to saving the Scoobies from Glory and the Knights of Byzantium. However, this is an increase in her ability to access her potential, not an increase in her potential itself. Tara doesn't gain this knowledge, both because of a lack of opportunity when this happens and because she has a different philosophy about using magic, so we don't know whether there's any difference between Willow's potential or Tara's.
Is magical power strength, the potential to cast particularly powerful spells or simply more spells, or knowledge? While knowledge can obviously change, it's not so obvious that strength can change, though magical strength can be borrowed from another person (Tara and Willow's shared spells) or an artifact (Jonathan's magic bone.) Is strength something you develop or something innate? If it's innate, it seems that everyone has it since even Xander can cast a spell by speaking Latin in front of the books. Perhaps power is a combination of strength and knowledge, innate and learned abilities, making it almost impossible to simply declare who is the more powerful caster.
Since I've illustrated the problems with determining who has the greater magical strength or potential to cast spells above, what about the question of who has more knowledge? Again, the answer isn't clear. Tara has a narrow but deep focus in a single tradition learned from her mother, while Willow has a shallow but wide knowledge of various spells that have come in useful to the Scoobies with her cookbook approach.
Willow clearly knows more spells that are useful in a fight by the end of season 5, but she still has the problem with her spells going awry at times. On the other hand, Tara doesn't seem to know as many spells, but her spells reliably work and she has the ability to see auras. What does Tara's reliability mean compared to Willow's unreliability in terms of power, and how does flashy combat magic compare in terms of power to quiet informative magic like Tara's ability to see auras?
Should Willow be more powerful than Tara?
This is a more subjective question than my previous two, but I think it's worth considering. My answer is no, because making Willow more powerful in terms of the story means focusing more strongly on Willow as a witch, which is a mistake for several reasons.
First, it harms the character of Tara by making her redundant to the Scoobies, all the more so if she isn't as useful in terms of slaying as Willow. There are fewer reasons to show Tara onscreen and even fewer reasons to keep her as part of the cast at all if the only thing that makes her special is that she's Willow's girlfriend. They didn't keep Kendra, a weaker Slayer, around as a regular, though they did keep Faith because her attitude towards Slaying made her an important and interesting contrast to Buffy. The idea of a contrast between Willow and Tara played up to the level of the Buffy/Faith dichotomy could have been used in season 6 and indeed, we do see hints of this idea, but it's never fully developed.
Second, it's harmful to the character of Willow because the more Willow focuses on her magic, the less focus we see on her intelligence and academic abilities, to the point where viewers forget that Willow's smart. I can understand that a new ability like magic could make Willow act less mature, but it shouldn't make her stupid and that's the really frustrating change in Willow after high school. We're still told that Willow's smart, but we're no longer shown that this is the case as we used to see in episodes like The Dark Age with her plan to deal with Eyghon. Dealing with the Initiative and Adam would've been a great opportunity to show Willow being smart and scientific, with Tara as magic or research support, instead of defeating Adam with a big spell that came out of nowhere at the end of the season.
Finally, the focus of the show is on the Slayer, as the title clearly indicates, which doesn't leave room for a regular focus on witches, particularly when one of them is more powerful than the Slayer. Having two characters whose focus is on casting spells with only one Slayer means that at least one of the witches is going to get shortchanged in terms of screen time because the show has to focus on Slaying. If Willow has a different role in terms of the Scoobies than Tara, then there's more room to show Tara. Tara could be the one that Willow goes to when the Scoobies need a spell like she did in season 4. It's also worth pointing out that the spells in season 4 focused on information, defense, and transformation, all areas which don't detract from the focus on the Slayer's combat prowess.
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"Omnia mutantur, nihil interit." -- "Everything changes, but nothing is truly lost."
Let me jump to the first questions, or at least one of them. Who is more powerful? I agree that canon clearly dictates that Willow was more "powerful". Why powerful in quotations? Her actions and their results were more readily apparant in the violent world of living on a Hellmouth. In my heart, as a Wiccan, I believe Tara was actually more powerful. And perhaps for the wrong, or maybe it would be better to say for more frightening reasons. I think most would agree that Magick is religion or faith to Tara. Turn on CNN. What are we doing to each other, what are we doing to ourselves in the name of faith and religion? I used to run a chat room where I asked just that of every person who came in. What are you capable of? What would you do in the name of your faith? TO what extent would you follow your religion? I believe I can also say that Magick was Tara's greatest bond to her mother. We never found out, never even got a glimpse really of what Tara was capable of in the name of the faith her mother taught her.