I've had a chance to get an idea of what kind of missions pay off the most.
Patrol Missions have a really really good pace. I think I enjoy those the most, because it takes 10-20 mins to finish a system patrol. To me the pace is good, its solid, I like going in, getting it done quickly, and getting out.
Episode Missions on the other hand.... Take as long as 40-60 minutes. Give or take 10. I start in space, then get sent to ground, then back up in space, back to the ground, then finish it off back in space. While I enjoy the unfolding story, I can't help but feel like the time it takes outweighs the reward at the end. (i.e the ends don't justify the means)
Apparently they are repeatable. However I have found once I get the ones where they ask me to deliver medical supplies, I end up dropping the mission and moving onto something else. Why? Because I wasn't sure how to continue. I assumed I had to go somewhere, buy the supply's and fly back to deliver. So i ended up dropping the exploration mission and moving onto something else because it seemed like a waste of time.
Since Im a Engi/Cruiser pilot, my DPS sucks, and I'll never benefit from fleet actions as I'll never see the top of the list, did it once for the exp bonus, and moved on.
Deep Space Encounters
Haven't done much of these either, by far the grindiest missions in the game. Did it once, didn't do it again.
All in all, the patrol missions I have found to be the most enjoyable.
For the steady pace and for the decent Exp. (Wish they were repeatable)