Hi you mentioned, "train" are you planning to use this on a daily commute?
I have both and have commuted in London for years. To be honest it's a no brainer if you are on trains. The touchscreen on DS is not perfect precision to start with, you add in the moving trains and it is not much fun at all tbh.
I can't think I am alone in this, every person I see on the train with a handheld, 90 % are PSPs and he ones that are DS are usually being played by a child with a parent on the train etc. I found playing games like Yoshi's Island, Warioware very hard to play whilst on a moving train (granted those 2 rely on touch screen interface a lot).