Hoo boy!
That's like comparing apples and... hand grenades. Sure there are some
*basic* similarities (like the core menu items in all MS products, but
there's a LOT more to contrast than compare between those two apps.
Different roles, different scopes, different skills required.
That said, I love Visio and I'm a huge proponent of the program, so if
you're into diagramming, illustrating or charting at all give it a shot.
Try a trial version first to see if it suits your needs, then buy a book and
go for the full version if you're happy.
Randall Arnold