And either way, you wont' need a new battery.
My original DS (not Lite, the original "brick" DS) I bought back when it first came out in 2004, and it still runs just fine. Sure, the battery life is about half of what it used to be. But it doesn't really matter, because I don't play it all that much since I have a 3DS.
And yes, I played that daily too. Ran it down, charged it up, ran it down. Or just left it plugged in all the time. Took it everywhere.
The last thing you should be worried about is the battery going so dead that you can't even play while plugged in.
so its not like the psp well sony screwed us good then
The days of the user-replaceable battery are pretty much over. With new battery technologies, it's probably not even necessary. If the battery does die, and refuses to take a charge, you can send it to Sony (or an electronic shop of your choice) to swap out the battery (I haven't been able to find a suitable replacement battery to replace it with, so there's not really a way to DIY).
My wife has a 5 year old iPhone 3GS that still accepts a charge, fully charges and lasts 3-4 days between charges when she uses it regularly. The battery in the Vita is a couple of generations of Lithium-Ion battery newer, as well. The days of the old, unstable Ni-Cad battery are long over.