Canon 70-200 IS Mk1 for $1k or Mk2 for $2K?

Hey all!

I'm getting ready to pull the trigger on a 70-200 IS lens to use during some motorsports events and general shooting. I've found a Mk1 lens for about $1k, which is well within my budget, but the Mk2s I've found are roughly twice that cost and a bit out of my price range.

I know that the Mk2 is vastly improved over the Mk1, but to those who have used both, what would you do given these price points?