organic chemistry - Is oxygen with a positive charge more stable? (comparison of canonical structures)

Which of the contributing structures of the resonance below is more stable?

I'm watching a video lecture by a professor of my college where he puts this question to the class. The class unanimously says B. At first I thought they were wrong, but then the professor agreed with them.

Is this true? The reason he gave was that oxygen's octet would be complete in B, hence it is more stable. But I don't understand this.

1) Isn't the octet complete in A too? It has 2 covalent bonds - 4 electrons (2 from sharing) and 2 lone pairs. Adding up to 8.

2) Oxygen being more electronegative than carbon, would pull the electrons towards itself, leading me to think that it would be more comfortable with a lone pair rather than another bond. Also, won't it prefer to lose out that '+' charge which the bonding's burdened onto it?

Am I wrong?

Another thing which made me believe that Prof. may have made a mistake while talking about the more stable compound, is that when he added Br$^-$ to the compound, he added it to the compound A. And don't you continue reactions with the most stable products?

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Answer

The presence of an extra bond in the canonical structure B, along with the completion of the carbon valency makes it more stable than A. Br$^-$ will attach to A itself, though, because the electrons on oxygen will repel the incoming nucleophile.
Thanks, everyone!

Answer

Which of the contributing structures of the resonance below is more stable?

Technically, neither. Both structures A and B are resonance contributors to the same true structure of the ion. Structures A and B, because they both represent the same species, cannot have different energies, and therefor they cannot have different stabilities.

What the question is asking is "Which contributor is more important in describing the structure and behavior of the hybrid?" Contributor B probably approximates the structure better (i.e. the positive charge is more on the oxygen than on the carbon), but structure A represents the reactivity better (i.e. nucleophiles want to attack the carbon and not the oxygen).