In that case, shoot both slide and C41. You can overexpose the C41 and it'll still be good so that takes away some of the timing and metering stress. And if you're scanning, you can fix any number of dynamic-range issues in a C41 neg that you can't fix in a chrome. I'd probably shoot Reala* (or better yet, Portra 160 or Fuji Pro160S if you can get them) due their lower contrast for the C41 option, though Ektar will be sharper and have more pop to it.
But if you nail the chrome, then that will be awesome.
* I was sorely tempted to reply with just "Reala, duh" when I saw this thread after the previous two