Depends a bit. Guirassy right now likely gives you more bang for the buck and is a more experienced and proven player. But he is only useful as competition for Hojlund.
Beier is still more of a talent, he would surely have the higher resale value even if it doesn't work out well at United. And he can play a second striker role around Hojlund when needed, you can easily sub him on if you are chasing a goal.
Besides that both have the most important experience for a player coming to United: They know how a chaotic shitshow of an underperforming former big club looks like (Stuttgart and Hannover).
Both would be good signings, but I feel like Guirassy really likes being the main man at a club, I'm not sure he would agree to a "bench contract".