That theory is misinformation supported by knife sellers who want to be able to sell forged knives at a higher price. A blade's ability to hold an edge depends on its steel, and steel's ability to hold an edge depends on its composition and how it's heat treated, not by whether it's stamped or forged. And as for steel composition, Global uses CROMOVA 18 steel and Zwilling uses X50CrMoV15, which have very similar characteristics, and I can't say one is any better than the other. As for heat treating, the point is to make the steel hard enough to take and hold an edge without making it brittle enough to break in normal use. The measurement of steel hardness is the Rockwell scales. Global hardens their blades to 58 HRC (Hardness, Rockwell scale C). Zwilling hardens their steel to different extents for different models, but the Pro is also 58 HRC. So the only real differences between the Global and Zwilling Pro is the geometry (overall shape, most importantly the blade profile and handle shape). Handle shape is almost irrelevant if you hold the knife in the pinch grip (which I highly recommend for comfort, control, and avoiding fatigue). So then the biggest difference that will matter in use is the weight of the knife, and generally, forged knives weigh a bit more than stamped, which makes a positive impression when you first get into using serious chef's knives, and then becomes a negative if you go on to use the knife a lot. For the home chef who only clooks a few times a week, that doesn't matter nearly as much - just use whatever you enjoy using.
EDIT: Possible correction: I took the 58 HRC of the Zwilling Pro from here:
http://www.metrokitchen.com/zwilling-ja ... ison-guide
But then found other sites saying it's 57 HRC. If that's true then the Global is slightly harder, so theoretically would hold an edge slightly better. However the difference will be irrelevant if you give it a quick hone with a sharpening steel before each use, which is highly recommended because it greatly extends the time between sharpenings. (Not getting into honing vs. sharpening except to say that a sharpening steel is a hone with a textured edge that does remove a small amount of steel as it hones.)
Another edit since people still seem to be reading this: Zwilling's own website states that the Pro is HRC 57, so I'd go with that. Not a big factor when comparing to the Global. The biggest factor would be the blade profile: the Swilling has a lot of "belly" - lots of curve, very little flat spot. (And this is only made more severe by the shortness of the 7" blade.) The Global has a lower tip, less curve, and a longer flat section at the back, which makes it better for tap-chopping or push-cutting (where you move the knife straight up and down, no rocking, no slicing action - the fastest way to chop celery etc.) because it has a longer section of contact with the cutting board, so more food gets cut completely through in each chop. Both shapes are good for rock chopping and slicing.