So long as the Cervelo is in really good shape, I say go for the P1. Absolutely nothing wrong with 10-speed. On a flatter course, you're going to use less than half of the gears anyway.
If your experience is anything like mine, it will take 6+ months to get comfortable in that aero position. I'm actually still fiddling with my setup-- yet another seatpost on the way as I type this.
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