Depends if you are just wanting a game system and already have a smart phone you are happy with. Seems two different things, in spite of the Play being more game centric than other phones.
That said, the 3DS is getting a bad rep real fast for battery life, which is a first for Nintendo, who are known for good battery life in their products. Not to mention the game development appears to be mainly Nintendo and only a smaller than normal list of main devs. Most commercial devs have moved a bunch resources on to Apple and Android dev (mainly Apple). The higher price and lack of games is hurting 3DS sales (at least in the US).
I am an old school gamer, so the Play is the better choice, even if using the Play for just games. No comparsion (IMO). added: I will still probably get a 3DS when the price drops $50- mainly for Dead or Alive and Street Fighter 4.