On for example, for instance

A correspondent writes to ask whether there is any difference between for example and for instance.

I think most people use these as stylistic alternatives, to avoid repetition, without any difference in meaning. The OED glosses for example as 'a typical instance' and for instance as 'for example'.

Differences...? For example is older - first recorded usage in 1447. For instance, 1657. And for example is much more frequent - about five times more so, in some corpora. Also, their distribution isn't identical. The expressions have developed further usages where the words don't easily substitute, such as I'll give you a for instance and by way of example.

But am I right to feel that related senses of the two words could influence the selection? The original use of instance (as in 'at the instance of', and related time-related words such as instant) conveys a sense of urgency or earnestness. Perhaps it's the phonaesthetics of the two words (the contrast in stress position and vowel height) which makes me think I would use for instance when I want to be a bit more emphatic and for example in a more leisurely exposition. I'd be interested to get some other opinions on the point.