You'll be able to access the DS and 3DS library. At $170 it will definitely be worth getting. --- I can't wait for Super Mario 3D, Mario Kart 3D, Paper Mario 3D, Luigi's Mansion 2, Kid Icarus: Uprising, Kirby Mass Attack, Kirby Wii, and Skyward Sword!
Well seeing as you're comparing it to the DS, in my opinion the 3DS is currently way worse than DS. Maybe it'll improve, but I don't recommend you buy it right now unless you really want it, because you may be disappointed. That said I love my 3DS!(I know it sounds weird :/)