I seriously don't understand why are we comparing a 3DS and a smartphone in the first place. We were asked to tell Tarvos should he buy a 3DS or a smartphone, but both are in-comparable. 3DS is a 3D with out glasses, cheap portable gaming console and a smartphone is a touchscreen swiss armyknife of electronics that is often quite overpriced.
Also, just because you can game on a smartphone doesn't mean its going to be fun. Onscreen controls suck.
If you want to go for a cheap portable enjoyment device, choose a 3DS (preferably the XL version). When you have a smartphone, you have to pay a contract for the phone and a contract for free internet browsing, both costing more than 10$ a month + the expensive phone with many features you wont use.