I've got three 12 string guitars.
Guild D212 is kept tuned to D (two half steps down) for tone and for ease of play, and to preserve the neck angle on this 41 year old guitar.
Guild JF55-12 is tuned to D# (one half step down) for tone and ease of play.
Taylor 856 is tuned to either E or D#, depending on what I want to hear.
Regarding strings, sometimes they can surprise you. I recently put a set of 9-46 strings on one of the 12 strings, and they were not appreciable easier to play than the 10-47 set that had been on it previously.
Having said that, I am a big fan of reduced tension strings. For my 12 strings, the favorite is the 11-47 set of Newtone Heritage strings, ordered directly from the factory in England (just google Newtone Guitar Strings). Ease of play is so important to me, because I don't want to struggle playing barre chords up the neck.