Unless you are developing a full-blown app (such as WoltLab Suite Forum or WoltLab Suite Calendar) the first one would be equivalent, because it defaults to WoltLab Suite Core for non-apps.
Thanks a lot for the info
Starting with WSC 3.0, you can also ommit the filename:
<instruction type="file" />
This instruction searches for a files.tar and extracts it's contents in WoltLab Suite Core.