GET A DS LITE..
1) Battery Life last for 15++ hours on lower brightest settings for each full-charge of your battery.Which probably takes only 3hours.
2) Flash carts for DS are also available..Look for "R4DS" , can find them in SimLimSquare , but becareful , lots of uncle there like to cheat money.. The price range for R4DS should be around $50 Only
3) DS Lite has wifi-connection , means can trade pokemon over the internet
Choose yourself , but i definitely go with DSLite , and by the way , an 1Gb microSD can hold around 15 DS games..