You don't want to use the first way, unless indirectly, by redefining the meaning of all font size changing commands:
The reason is that any command that internally uses
\normalsize (or another command of the series above) would spoil your hand crafted leading. What are these commands? To begin with, there is
\normalsize; not to forget that it is issued during the output routine, for setting headers and footers.
Where do the definitions you need to change reside? In
.clo files; for the
book class they are in
bk12.clo, corresponding to the initial option given to
\documentclass. For other classes there are different files (you can see which one is loaded by examining the log file).
The method with
\leading are equivalent, the second is perhaps easier to understand.
\fontsize commands in the document is not recommendable, in my opinion.