I would say Leviathan was better. Had a little side investigation portion and was related to the overall plot. Omega was just a giant sidequest with nothing unique about it. It was fun because ME3 is just fun, but it was nothing special.
"Hairy Zeus on a traffic light!" -Deadpool, Issue #3
I'm a Mass Effect junkie and own all dlc across the trilogy, so I am not sure I can answer this question objectively. Well actually, may be more of a Bioware junkie as I did the same thing to Da: O/Awakening and Da2 with their dlc.....
There is no justification to the $15 price tag, and if you get it on Xbox, it's actually $20, unless you had points left over. I was really disappointed to hear MS was doing away with MSPs, except for Live.