Depending what your sources for meat are, cooking a flat can be almost the same as cooking a packer. If the flat is, essentially, simply removed from the packer and left alone - so it is still of good size, still thick, still untrimmed - this is usually the case.
In many areas (like mine in Florida) flats in the condition I just noted are impossible to find. There, all flats are wildly overtrimmed and thus substantially thinner. Sometimes the thinnest end is also removed, making them small, sad speciemns of what they once were. These do not cook the same as a packer at all, let alone cook the same as a good flat.
If you can get nice, thick flats that are untrimmed or trimmed minimally, go for it. If not, I'd suggest getting a packer and either cook as is and freeze the point after the cook for use in future meals (this is what I do), or separate point from flat before cooking and either freeze the point raw for something in the future, or grind it for burgers, then freeze, and cook the flat alone.
