Is there a strong argument to use shell sort over insertion sort on an array of 10 elements
It's an empirical question. First you need to come up with a probability distribution on arrays, for example the uniform distribution on all permutations of $1,\ldots,n$. For each $n$, you can measure the running time of both algorithms, and see which performs faster. The threshold 10 in reality depends on the implementation and the CPU, and is quoted only because it's a "nice" number, functioning as a rule of thumb.

So far I have assumed you're interested in minimizing the average case running time. If you're interested in the worst case running time, you'll have to run the algorithms on all permutations, which will quickly become infeasible.