= Solution
Potassium is electropositive and transfers <electron density> toward the metal. The resulting surface dipole generally lowers the metal <work function> at such submonolayer coverages, reducing the tunnelling barrier and its inverse decay length $q\propto\sqrt{\Phi_{\rm effective}}$.
At the same specified $5\,\mathrm{\mathring A}$ separation, a smaller $q$ gives a larger $\exp(-2qs)$, so the expected tunnelling current is \b[greater than $1\,\mathrm{nA}$] in the barrier-dominated comparison. No numerical value follows without the modified <work function> and electronic <density of states>. A real STM image also depends on the local <density of states> and adsorbate corrugation; equal geometric separation does not imply equal electronic barrier height.
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