I've had a DS Lite for almost a year now. I would say go with the upgrade if you can. Try and get something for your "phattie" though. Every $$ counts. I was able to trade my old GB Advance for approx $50 towards the Lite.
As for colour, I don't think it matters. I have black to go with my PSP. My nephew has white and it looks dirty all the time. My daughter wants pink.
I would suggest getting some sort of screen protector to avoid scratches.
These are my thoughts.