Crazy! We're pretty much the exact same size - other than me being 5'9" and a half, but we have the same inseam.
I ride a 54, as i have short legs. As the other person said, older 105 is going to be better than anything on a new bike, such as tiagra, etc.
get a close up of the cassette and chainrings on the giant, and then if you got it, check the chain stretch, and replace if nessassary. if you dont replace it, it will ruin your cassette and chainrings, and things get alot more expensive.