Copy As Link works better for me because it allows me to have some shared source
files as well as some source files that are not shared, without having to change
filenames. With file linking, two materially-different versions of the same source
file having the same filename can live in two different project folders, but can't live in
the same folder shared by two projects.
Here's how I've done multiple projects targeting multiple releases:
MyProject < solution folder
MyProject17 < R17-specific project and source files
MyProject18 < R18-specific project and source files
Common < Source files shared by all projects, added as link to each
The Common folder contains all source files that are shared by all
projects, and do not require changes. If at some point, I need to
fork one or more of those files, they come out of the Common folder
and get copied to each project folder, and added to each project.
Aside from that, there isn't really much difference between copying
projects and adding files as links, and hence, not much else to debate