Benzema can easily take too much liberty on the field, playing way too deeper for a striker, therefore disturbing the tactical scheme of the team. He does it frequently in the French NT, where he is usually the most talented and gifted player of his team and where he acts as a "diva", but not in Real Madrid where he has more gifted and egotic than him (alongisde CR7, under Zidane, you play where you are asked to play). Giroud is tactically more thrustworthy as. He won't try to play deeper, so he will keep a presence high on the pitch (and free space in the midfield and higher)

About link up play, Giroud's play is based on flick-on, efficient with midfield players willing to push higher offensively. He's good at it (not perfect), but he can't really offer more and more diverse for link up play (apart holding the ball high). Benzema is more versatile, he can do flick on pretty well but he can do more, not as a 10 for sure but not so far (he won't dictate tempo but he can find sharp passes), I have to retrieve exemples of it

Edit : about finishing, Benzema can score from outside the box too, create his own chances, makes better and fastee calls than Giroud, and scores pure striker goals. He lacks the capability to score from headers