In my opinion, they're virtually identical. In both ranges, be careful what you order however ----- typically, unless you're Ikea or a tract-home developer, you want the models that allow adjustment in all three axes --- however, I think both companies offer models that delete certain adjustments, as well as models that have those adjustments, but require loosening of the mounting screws to adjust. The top models have (3) adjuster screws (cam/jack) that let you dial in door alignment.
Grass (i think...) is more likely to have cast parts, while all the Blums are stamped to my memory. The cast ones look a little nicer, but honestly, all the euro hinges are hideous. They also offer different style door attachments -- while each has the 32mm cup, some have expansion dowels to secure the door panel, and some other production-level trickery. For custom work, I think the basic #8 countersunk screw flange is the best.
Last advice, and most important if you don't already know ---- DONT USE A PHILLIPS! All these hinges have Posi-Drive screw heads, and once you learn that your hinge life will improve dramatically! Sure, they look virtually identical, but its shocking to witness the torque / cam-out difference.