£109.99 compared to £139.99 3DS and £164.99 3DS XL, the equivalent of one or two new games - mind you, considering all the promotions that Nintendo have advertised, you could potentially make up that difference anyway with the 3DS XL (free Monster Hunter and Animal Crossing download versions if you have a friend willing to give you a Recommendation code
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Considering the whole not being able to fold it thing, it's a smart move by Nintendo - the hinges are most definitely the weakest part of the system (one of the hinges on my original DS snapped after 8 years), so it's brilliant there's no chance of that happening on the 2DS. Plus, it comes out on the same day Pokémon X and Y is released, so they're capitalising on the fact that children will want it and parents will take the opportunity to buy the cheapest console to play it.
As to it being portable, people take iPads when they're out and about, not to mention the size of some touchscreen phones have become colossal recently. All you need is the carry case and you're set.
The main reason the GBA SP was better than the GBA was because of the backlight - the folding shape helped protect it certainly, but the real innovation came from being able to play games without having to angle your GBA towards a light source
Unless you're planning to take the 2DS to places / hand it over to small children that will easily scratch the screens, the 2DS is fine as an alternative to the 3DS, it's not like you can easily fit the folded 3DS into your pocket anyway.