It comes down to whether developers will be able to harness the NGP's power. My guess is that it's overkill, and that the NGP, like the PSP, will fail to live up to its full potential. I'd be tempted to say that the NGP's extras (ie. 3G, etc.) are just a ploy to help draw consumers in, but I don't know enough about Sony's sales strategy to say anything.
Of course, the 3DS could also face the same issues. It is releasing at a price point far higher than that of the DS, or even the Wii, and its battery life is only fair. But Nintendo could easily remedy that issue. One upside is that developers will probably be less likely to rush products out, but we won't see too many new IPs, either.
Honestly, I don't feel like the NGP has a large chance of mainstream success. Its game library will likely be a mashup of PSP and PS3 franchises (ie. very little new material).
Personally, I feel like the 3DS could hit anywhere between 75-110 million WW, or possibly DS numbers if something big happened, but I don't ever see NGP passing 100 million, even if everything went right with the pricing and all. I see more like 50-60 million.