I have both a regular DS (or as the cool kids say, the DS Phat) and a DS Lite. I got the DS on launch day, but I wasn't in a hurry to get a DS Lite. But eventually (like a couple months later) I got one.
Well, we were looking for a second DS because my brother and I wanted to play some versus games. Well, we never did (and still haven't done) much of that. But we knew eventually there'd be a Pok?mon game (or rather a pair of them), and we didn't want to get two of the same model of DS just to play them, that'd just be redundant.
Well, I'm rambling here. Anyway, I'm sure you've seen them umpteen images of DS and DS Lite comparisons. I can personally vouch for the upgrade, if you only need one, then by all means trade up.
I did Brain Age on the regular DS last night. Ick. When you get used to the DS Lite, you will be surprised and how incredibly bland and dim the DS Phat's screens are. And I noticed this on Brain Age - not a very colourful game at all! If you play something like the Mario, Pok?mon, Kirby games, etc., anything really bright and colourful, you will certainly wonder why you stuck with the original DS. Plus, the Lite is sleeker and smaller! But you already knew that.
The Start and Select buttons are a little small, but seeing as how you won't use them much that's easy to ignore. The Power Switch is on the right side as a switch instead of a button...it's rare to confuse it for the volume switch but I've had a few little scares when I forget what I'm reaching for - also, it's next to the stylus slot. >_< A DS Lite I first got had a red dead pixel in the middle of the touchscreen, but the store took it back with no problem. But now my DS Lite is scratched up more on the touch screen, but that's likely because I use more than the original DS. And why wouldn't I? When Pok?mon Diamond and Pearl come out, me and my brother will probably fight over who gets to use the Lite.