I'd be curious to know how often the closest pair has $\min(m,n)>2$. Bennett shows that $|a^m-b^n|<\frac{\max(\sqrt{a^m},\sqrt{b^n})}{4}$ happens at most once. However this might not happen for most pairs $a,b$ and even when this happens, it might not be a min. (And it seems rare for it to happen with $\min(m,n)>1$.) A rather spectacular (to my mind) case is $13^3-3^7=10$. The first few convergents of the continued fraction for the ratio of the logs are $1/2,2/5,3/7,239/558$ This shows that 3/7 is an extremely good approximation (of course). But the closest pair is $13^1-3^2=4$.

**later** I looked for instances of $|a^m-b^n|<b$ with $2 \le a \le 99$ $n\ge 3$ and $m<200$ (also $a<b$ and neither a nor b a power of a smaller integer ). The only instances are $2^7-5^3=3$ and $13^3-3^7=10$. Both of these happen to have $|a^m-b^n|=b-a$