Because I'm a Wizard, not a Witch. I remember once it being stated that in some cases, wizard and witch have taken on gender stereotypes that haven't existed before. Its very unusual for a female practitioner to consider herself a wizard; witch has been rhetorically programmed to be identified with 'woman' for so long, and through so much torment and heartbreak, that to identify oneself as a 'witch' gives a woman power to overcome trials. Therefore, though witch and wizard might have no linguistic basis in gender, the historical implications usually have as much impact on what a person chooses to call themselves as anything.
In addition, the ideas of witch and wizard have connotations about the kind of 'magick' one is likely to delve into. Witchcraft is very much associated with the here and now, finding its metaphor in nature magick, which is why the study of herbology, potions and whatnot is so strongly associated with the study. Wizardry, on the other hand, is associated with the larger forces of existence, ideas, the nature of existence itself, the 'ur' if you will. If you think about it this way, this also explains how both a witch and wizard might approach a similar problem: a witch might give someone a 'charm' to help them find a job, while a wizard might simply 'peer' into the maw of existence, 'see' how the universe is unfolding, and use such information to either guide the person to where they should be, or give them moral support, knowing that what they are going through now is going to pay off for them later.
However, if we are dealing with matters of magickal application, then the definitions of witch and wizard begin to take on a more pointed distinction. In its most basic understanding, a witch is someone who practices their craft using a wide variety of methods: spells, charms, potions, summons, incantations, to help augment the force of their will. A wizard is someone who practices their craft by force of will alone. In other words, a wizard is one who does not draw a pentagram on a floor with the magickal words written upon it in order to cast a protection spell: they simply 'will' protection into being. This distinction can be drawn further in that the more 'wizardly' one becomes, the less likely one will ever turn to using the tools to help themselves. Relying on the force of one's own will exclusively is what makes a wizard stronger, and most wizards will do their best to do their work with the least amount of 'support' as possible, since such hardship is the only way they know to make their will, and thus the force of their magick, stronger.
This arguement is sometimes seen in sci-fi and fantasy books as the distinction between a wizard and a high wizard or low magick and arcane or high magick, the distinction being that it is much more difficult to cause affect without using a focus or a complement (a boiling cauldron, roots, herbs, fairy dust, elder wands, etc etc) and therefore making the 'magick' all the more vulnerable to being discharged or broken.
Some of this is gender-bias, since the province of wizardry throughout the ages has taken on a more male-oriented role, but many such 'wizards' are closer to witches on further examination, since they often use props and other magickal implements (much like a witch) in order to perform their deeds.
As for the affectiveness of either method, neither is more or less effective than the other, since it is entirely dependent upon the practitioner. Just as a poorly trained martial artist can be beaten by a very skilled streetfighter, so too can a poorly developed wizard be overcome by a well-practiced witch, and I am not distinguishing by gender now, simply abilities. The 'power' comes from the person, and not by the methods they use.
"One is not born a woman, but becomes one."- Simone de Beauvoir