To the average consumer, the price factor may hold a fair amount of sway, but I'm currently convinced that the entry price will be $299 and probably won't be the deciding factor in the long run for most.
It's going to boil down to developer support and the games. Each platform will have its respective audience with a some overlap in between, but not in the form of dual platform ports due to the different design approach each system uses.
I'm really not sure where this whole "two screens is the advantage of the DS/3DS" argument came from though. The design basically came about in the interest of practicality; it's difficult to play certain games that require touch screen input if your finger or stylus is blocking on screen action. Hence the bottom screen for touch input and maps/menu interface. Plus I'm guessing the design might have been more economical from a production and parts sourcing standpoint to use two low res displays rather than a larger high res display.
Either way, the average DS game didn't exactly demand a two screen set up even though there were plenty of games that were well designed enough to take advantage of it.
As far as the NGP goes, if a game required a second external display to play properly/non-gimped, it would be dead in the water, even if SCE pushes any such dual screen compatibility as a key feature for whatever reason. While it would be a neat feature, it would mean anchoring a "portable" platform game to the home for all intents an purposes. And a second touch screen display isn't necessary as any action game where touch input could interfere with the visuals can easily be designed to use the rear touch panel instead. It serves the same purpose and all you're really giving up is an additional set of static menus, which are typically a HUD in console games anyway.
In the end, the future of portable gaming really isn't a head to head competition between the 3DS and the NGP, it's more like a competition between dedicated gaming devices and smartphones. Which will generate the most revenue with the most reasonable development budgets for the typical developer and offer games at the most reasonable prices for the entertainment value they provide.
In a way it's the $1-5 games vs. the $20-40 games. Sure the cheap games aren't typically going to be something the average consumer plays for dozens of hours, but how many paying customers (those who aren't currently pirating DS or PSP games) are actually getting that kind of play from their full priced games?
