The split line, with 2mm offset and delete face, would work fine in most cases, but it depends on a few things. First, does the door wrap around (say along the roof line or rocker panel), which might make creating a consistent gap difficult with your average split line feature. In this case, your second idea of the ruled surface, offset surface, and trim would work much better. This also allows you to get a fillet along the profile on both the body and door, which will render much more realistically.
If you don't have the wrap issue and you aren't looking up close in a rendering, then the offset split line and delete face might look just fine.
I'm sure there are other ways to approach this, but these 2 methods seem somewhat acceptable.
These doors are usually on curved surfaces so I'll go with the offset ruled surface for now.