Is krypton gas more energy-efficient that low-e argon?
As someone who bought new windows a couple years ago and researched it to death, almost anything a window salesmen tells you will be a marketing gimmick.

The fact of the matter is that when it comes to heat transfer resistance with windows, you reach the point of diminishing returns rather quickly.

Triple pane windows will give you a slight advantage in energy efficiency however they tend to be heavier and far more expensive.  The extra weight can often cause them to wear out the casing or unseat over time however.  Now consider comparing these to dual pane windows with a low E coating and you will only see a slight drop in resistance from the window itself.

Both Krypton and Argon are noble gases and both achieve the same effect, they insulate between 2 or more panes of glass by slowing down the transfer of heat.  Krypton is typically used in triple pane where Argon is typically used in double pane and this has to do with the maximum heat transfer resistance for each gas at different widths between panes of glass.

Krypton does in fact do slightly better but usually at an enormous upfront cost that in my opinion would be hard to get ROI on over the years as compared to dual pane low E Argon windows.  Over time with the expansion and contraction of the casing and general exposure to the elements, microscopic stress fractures will start forming around the case slowly allowing all that expensive Krypton or Argon gas to escape, essentially making the windows lose heat transfer resistance over time.  Investing in a window with a better quality case design will do more to improve the overall energy efficiency of your windows than any investing in triple pane Krypton.

A good quality case design that is well insulated and uses high quality weather resistant vinyl will do more to increase the lifespan of the windows and maintain the energy efficiency of the windows by being more resistant to leaks and fractures that will result in the slow seepage of the noble gases.

It can be hard to seperate the gimmicks from the truth when it comes to window shopping, so thoroughly understanding the difference is important.  Figure out what your monthly heating bill is and use that as a basis for making a good decision.  If you are living in the Yukon and you have long frigid winters then perhaps spending the extra money for the triple pane krypton windows is worth it.