I've played on a DS Lite, the screen is fantastic, but I don't think the one on the normal DS is that bad; although I will say DS Lite looks a lot brighter. It doesn't really affect gameplay though, a few moments into any game and the brightness seems irrelevant.
Aesthetically it's a nicer looking machine, no doubt about it. I don't think it's quite as comfortable to hold or use though, but maybe if I had one it'd grow on me.
I wouldn't get a Lite as I don't think it's worth paying to upgrade, but if the brightness is important to you, go for it. I'd recommend a Wii as I think you'll have great fun with it, but if none of the games truly appeal to you it might be a bit of a waste.