This statement caused me to take a closer look at my logs. Whole raid this week, my average KS hit for 180k and average ES 215k (not counting crits, which show a similar ratio). Difference is a bit bigger than I expected, but of course it's not quite as simple as that considering shot shifting, charges, cooldowns, etc. In any case I think your reasoning is sound, regarding ES being better when it's free, and KS being better when it's not.
(Source:
http://www.warcraftlogs.com/reports/...4false%2453301)