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Service Cost is absolutely a factor for consideration. Premium Ships not only have a reduced service cost but also have a built in bonus to Credits earned, so it's not surprising to see the Atlanta pulling so far ahead of the tech tree Lightning.
Operations can be played at lower then Tier 8. While you won't have the same damage expectation, you will have an advantage from the reduced Service Cost which aids in rewards.
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Your Base XP being between 1100-1600 allows bonuses used to have more value per single use than in a Co-Op, but in a less stressful environment than PvP. PvP continues to be the area of highest-possible return, but it is a wildly unpredictable situation unlike Operations which provide scripted, albeit challenging content.