"But lux we need a sightstone for wards"
"I AM...THE WARD"
: 
How is that a decision? Doran's Ring for mid Lux, Spellthief's for support Lux. Easy.
I don't know, if I find myself playing Lux middle again for some reason, I might be tempted to go Spellthief's Edge. The vision and pick potential from Frost Queen's Claim could turn a potential ambush the other way, and the new speed enhancement could make it safer to proc passives.
+ 
no reason to start doran's ring
starter items are lame. real skill comes with dodging your opponent's shots without any items and still claiming first blood. which is why i hate tracking rounds.
though i always take duo lane income items. whether im support or the primary laner if im in a duo lane i take an income item.