Canon 70-200mm IS f2.8 vs f4

Quick question: Taking into account weight, IQ and general useability, which would you rather own? I'm trying to choose between them. Price isn't an issue (my wife has given me permission to buy the f2.8IS!), as lenses generally retain their value.

If it's relevant and people want to give me advice, I like shooting portraits. I have a 2x extender. I'm trading in a Sigma 70-200mm f2.8, and I'm looking for image stabilisation and better colour definition.

But given the diversity in opinions when this subject was previously discussed I'd appreciate one word answers: ie. f2.8 or f4.

Jon
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jonclayton.smugmug.com