3DS battery life is only poor when playing 3DS games. When playing DS games, the battery life is 7-8 hours. I have a first gen DS handheld, which had a battery life of 8-10 hours, and I can tell you that 5 years after purchase, the battery is still holding full charge.
7-8 hours is still pretty decent, I would advise you get a 3DS handheld. Paying $150 for a DS, for primarily one game is a lot. Yes you will get other Pokemons and handheld titles, but nothing new is coming out for the DS anymore. If you get the 3DS, you know Nintendo will make a new 6th generation Pokemon, and you're going to want that.
Also some of 3DS's games are looking to be quite good. Imagine playing Resident Evil with resolution almost as good as on consoles. Super Mario, Zelda remake, Starfox remake, Kid Icaurs, and Mario Kart are just the begining.
It would be silly to get a DS now. From my experience of buying a console at the end of life, you'll play it for 6 months to a year, and then put it down. Old gen DS games just won't excite you in a year, at least not as much as they do now.