Well, as Snake said, we folks in Australia don't get Infiniti, so I can't say i've seen both in the metal. I've seen the GS, and it's a very nice looking indeed. May be a little 'fat' looking because of its overall side profile, but nicely done.
..from the pictures the M45 looks good from the front, but the rear is just terrible, espcially the lights.
So my vote goes to the GS on this one.