Inconsistent letterspacing (interword space) in pdflatex vs lualatex

I see the same issue when using the Lucida OpenType fonts with LuaLaTeX, as well as with the Libertine fonts. There's another thread that gets into the fine details of what appears to be a small issue in LuaTeX. I found that I could gain control in LuaLaTeX by using Microtype's commands to adjust interword spacing, e.g.:

\SetTracking[
  spacing = {500*,100*,}, % choose values you like
  outer kerning = {*,*} 
  ] 
  {encoding = *}
  { 50 }  % choose a value you like

and then invoking \textls{\scshape{Stop stealing sheep!}. Microtype without manually increased interword the spacing seems to leave too little space between words when the interletter spacing is made rather large (e.g., 200/1000 em for some heading text).