In a D&M AVRs, enhanced should be selected if the cables can support 18gb/s. This is strictly a bandwidth setting, it doesn't affect video levels.Supposedly "enhanced" is the correct one, but from what I've seen from others on here, and in my own experience, "standard" looks noticeably better.
In the PJ, standard vs enhanced decides video levels, not bandwidth. Which is best depends on the source. Levels need to match, so settings should be checked with patterns. If you don't know, select auto which will choose the right setting depending on the source 99% of the time.
I prefer using standard, others (especially HTPC users who are also gamers or use their PC for non-video tasks) prefer enhanced. There again, it has to match if auto isn't used.
So there is not one recommendation that is best for all sources / situations re video levels, except auto if you don't know.
But re the AVR, it's definitely enhanced (higher bandwidth) for 4K content if your cables can take it, especially at 4K60p.
It is to limit incompatibility risks that the default in the AVR is set to standard (lower bandwidth).