Once this thread was about the December NPD numbers... but fast forward to Arc vs Natal and this is my take in succinct fashion:
Arc won't do well, Natal will.
Based on current info. Yes. We don't know what Arc entails. It's hard to imagine it doing well.
Sony also has the tradition of doing too much and saying too little. So by default, they will muddle up the picture and confuse everyone, including themselves. The software will likely be piecemeal yet again.
I still view Natal in a 'Guitar Hero' vein, where owners of existing consoles will buy it purely for the novelty factor, and non-owners will buy it potentially for the same. Should sell some more consoles for MS, and in its own right be a huge money-maker simply from its own sales+sales of novelty software. I don't see "women" as a demographic that will pile into this thing as has been mentioned, the accessory just has its own appeal. If the 360 gets a slimline version in the process, so much the better.
The Arc on the other hand... the problem for Sony is that the PS3 is not the "family" system, no matter what they do in that regard. Do the larger pool of PS3 owners care about 1-1 hand-centric motion mapping? I don't know, but I figure those that do already have a Wii, and those that don't won't care after its release, either. I'll have to see some pretty compelling stuff myself to take the plunge; who knows, maybe Sony has just the game waiting for me. For years I've been lamenting the lack of focus on their true casual/controller dark horse, the EyeToy, and honestly that Natal has come along ahead of Sony's getting there in spite of the hardware nature of the SPE's to be perfectly suited, the magic mirror demo of years ago, the Minority Report interface target vision... does it make sense? Someone at SCE HQ is not seeing the potential of some of the efforts that have already preceded.
I think it's inherently tied to their dated marketing model. I think they are still targeting a static 18 years old demography (like PS2). They are still using the hardware manufacturer's inventory/cost-based product approach (Slow stats driven, profit minded). They may be great at pushing large industry movement (like Blu-ray) and small, one-off CE devices, but ill-suited for fast-pace, high risk, consolidated platform.
A magic mirror app alone won't be able to cover their high cost.
Natal will equal something fresh, and Arc will seem like added system functionality to lock down an advantage from a competitor. Nothing wrong with the later, but the former engenders more excitement.
Since Sony pushed back their Arc rollout (probably trying to grapple with 3D, Arc, Sony Online Services, and other products), it gives MS a clear channel to market Natal all the way to Christmas. Meanwhile, Sony will be doing piecemeal 3D marketing, may be PSN/Sony Online Services subscription later, and then last minute Arc possibly with delayed games. They seem to think more means better (Ha ha). Should be just one simple concept all the way.
When they consolidated into NPD, Kaz Hirai should have dropped projects to focus, *not* pick up more projects. I think they will get their *ss handed back to them in eBook and PSP (Should have focus on PSP and PS3 only). The Sony execs cannot yell orders from far behind, and hope that the middle managers help them to push the strategies. They need to be in the front line themselves.
Now these are all personal opinion after looking at things from the outside, and having no clue what Arc is. So let's see if Sony can beat/kick their old habits. When we know more about Arc, perhaps we can see if they are still following old scripts.