Im making a list of all the matchups and if its best to take ghost or flash. This is what i got so far:
if they have an agressive jugnler and you want to stay alive pre6 
if you want to dance around him aftr 6
depends on rest of team
Thoughts and additions welcome.
dominusx1 votesI'd argue always flash vs Darius you need to instantly fuck off once he pulls you. + he has a bunch of slows and it's not hard to proc the passive before yo manage to get some distance on him. also you cant keep up with him if he procs passive and has at least one mr item swain is a bitch of a champ but Darius is good at dealing with bitches ;p but if you are competent you wont be inside Darius range so i dunno.
sirwaterfles1 votesYeah pre 6.24 i had no prob with dar as i just needed to play safe and could run him down with ghost after 6 but now i find myself wishing that i took flash a lot more as he wrecks me pre 6.
SNSD Sunnny1 votesyou pretty much always take flash on swain, its mandatory
sirwaterfles1 votesI have found that in matchups like teemo or shen you got a lot killpressure with ghost.
ChaddyFantome1 votesI go ghost/flash personally.
Ignite needs pointless/redundant and exhaust seems superfluous.