I imagine it would be like ME 2, you can make most of the important choices, but import a save will give you little more than that. For example if you use the DLC to choose the decisions in ME1, several minor NPCs will be dead no matter what you choose, but if you import a save, they will be alive and actually show up in the game.
So I'd say import save is the better choice and actually worth an play through if you have time. But I'll start using the choosing section during my second or third play through, since I don't have 5 different saves to import.