I was considering buying a XL and later on buying a second hand Lite from EB Games
Get a lite. It's cheaper, and it is reigion free, so you can import B/W.
When I decided to buy a new one not too long ago, I chose based on which colors were available and what battery life was available for each.
I hope I can help:
I chose the DSi XL because I already had both black and white DS Lites, and the other two neon colors were not appealing, so I chose the Red Wine DSi XL. I also chose the XL based on battery life. The DSi's power is substantially shorter in life than the Lite's, whereas the XL's is only about two hours shorter than the Lite's. So yes, even though it is bigger than it's regular predecessor, the XL has a longer usable time!
Get the cheapest and save up for a 3DS!!!!