i did answer same question looooong time ago, er... something like 1 year ago LOL
form my dig exp (well, i guess 2 years of digging wasnt long ya?) :
Gas n Spazial will give u quantity which are good for early artifact dig.
Oceanic will give u a quality which is good for dig UnCommon or Rare. even in dig time (i mean all the Common, UnCommon, and Rare dig time), Oceanic will still provide u ton of Common artifacts.
so, soeone might ask: `what`s the purpose to have Gas if Oceanic can provide a lot Common artifact while dig time?`
yes, the difference is here. when the dig time was passed, u will foudn out that Gas and Spazial will get artifacts easier than Oceanic but stil, most of them would be the Common artifacts. as Oceanic will still has better chance of getting UnCommon n Rare.