A few things to consider that haven't been mentioned 1) room size 2) available A/C power
Aside from that and as an McIntosh customer, my room is 13' X 17' with MC501's and Sophia II's. That is enough for me, it took a while to come to this point (to say 500w is enough) though. If I had the cash last year, I would have the 1.2KW's for sure.
Instead, I held out (ie, was broke for a time) and when I had the CC paid off, I went with a kick ass Pre-Amp, the C500T from McIntosh.
The reasoning was it improved the sound in a very, very big dramatic way as I was using a Classe SSP600 Home Theater Proc for 2 channel and for Movies/TV.
Buying the 1.2KW's would in no way shape or form give me that kind of sound.
With the MC501's in my room, the decible meeter hits 96db's and there is still room to go, there are probably 5-8 more db's left before the powerguard kicks in. At that db level, the "experts" say you begin to lose hearing and its true, but the hearing comes back to normal after a bit.
So my take is to buy the MC501's and spend the rest of the money elswhere in the system (speakers, pre-amp, source (maybe a DAC?))