Bash: using dot or "source" calling another script - what is difference?

The only difference is in portability. . is the POSIX-standard command for executing commands from a file; source is a more-readable synonym provided by bash and some other shells. bash itself, however, makes no distinction between the two.

There is no difference.

From the manual:

source

source filename

A synonym for . (see Bourne Shell Builtins).