You can compare the results and try to figure out why they differ in your spare time ...
Seriously, unless you need to combine features that only exist in disparate analytics tools, or you are a researcher who is after the nuances of the various tools, it would be foolish to link in more than one Web-based tool (I do it all the time ...). Their Javascript is not exactly light-weight. Tools that parse the server log have of course no run-time impact.
The metrics will differ because different tools count different things, because hit, visit, search engine spider, etc are not universally defined.