I've had both. The IS was actually rated safest car in class. Both cars are built rock solid.
The interior size difference is very obvious, unless your wife is very small, the GS is a much better fit for her and a baby. Also trunk room is bigger for more stuff.
IS 300 is a lot easier to "park" and see out of, GS C-pillar is a pita sometimes, lol
Good luck!

 

did you post the same question on the IS section? ofcourse everyone here is going to say the GS in order to realy get the feel for both cars, post on the IS section as well IMO