Currently whenever a user enters a link when no text is selected the first character of the displayed link is always capitalised. This is often incorrect, e.g. in the middle of a sentence or for articles like [[iPhone]] or [[grep]].
Especially until bug 50678 is fixed, correcting this is not at easy or intuitive.
So, at least until bug 48789 and bug 50945 are fixed and the display text is settable in the dialog, and probably beyond that, it would be best if the displayed link used the same case as the search term entered. i.e.
User entry -> wikitext equivalent of desired output
Green -> [[Green]]
green -> [[green]]
iPad -> [[iPad]]
HAMmer -> [[Hammer]]
london -> [[london]]
Yes, this will result in some wrong captialisations, but no more than at present and will only be temporary (as noted above)