All the above aside, there seem to be measurable differences in performance of line-in vs. mic-in signal paths of some ( /nearly all / all ?) recorders (available at avisoft for instance and sometimes found in published manufacturer specs).
I'm not qualified to evaluate why (which may vary from recorder to recorder) but my take away has been that (assuming the power supply performs better than the internal amplifiers) one is better off going line-in if the input level from any given configuration and setting allows it.