This is a matter of personal style, I think.
And personally:
I always prepare as much as I can.
I think stuff through (I'm a commuter so I have plenty of time for that) and write-up my thoughts as much as I have time for.
I tend to find that the more preparation I do, the better things go, and the better I am able to improvise on the night.
Cheers,
sjjh
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