I'm currently driving an 03 MCS and an 05 MCS and I can tell you first hand that there is a HUGE difference between the rear view mirrors on the two cars. In the 03, the rear view mirror is so small that I have difficulty seeing with it. The 05 on the other hand is MUCH larger and MUCH easier to see out of. I'm sure that if I only had the 03 mirror without the 05 to compare it to that I wouldn't complain, but after driving the two cars back to back I can really tell a big difference and the 05 is a lot better (for me anway).
Note that the 03 has a self-dimming mirror and the 05 doesn't, so in that respect, the 03 is actually a better rear view mirror. However, I'd take the larger size of the 05 rear view mirror without the self-dimming feature over the smaller self-dimming 03 mirror any day (yes, it is that much better IMHO).