Sound stage: The Grado is much more open, since it's open-back, and the stage is much wider than the K550, but in terms of depth they are both lacking.
Separation and detail: Grado is a clear winner. Everything is clear as day and well pronounced. The K550 is much warmer, though. The 325is can be pretty fatiguing at times, but I love it for aggressive metal. The K550 is much more laid back and with much fuller and extended bass. The Grado is bright, clear and punchy, but lacks that buttery kick drum kind of sound to it.
They both excel at different things and provide a different sort of utility and experience for me. I listen to the K550s when I'm on a long commute and need isolation at my desk. I use the Grado for more detailed listening in the office and while walking. I think the Grado is much more engaging and aggressive, while the K550 is a nice, relaxing experience. Both sound beautiful in their own ways. I'd say the K550 is much more comfortable and better for longer listening sessions, though.