All DS systems have the same resolution. DSi however has the best screen, IMHO - IPS, bigger, brighter
Chrono Trigger, one of my all time fav, looks great on a DSi XL:
My kid accidentally updated the FW on her 3DS and was getting an error on boot up. I had to update Luma manually by pulling out the SD card and replacing the files.
To prevent issues, I updated Luma first on the other 2/3DS we have and ran the system update.
I started buying 2/3DS pretty late, just last year. I've bought them on Craigslist/Facebook Market place locally, as refurbished from EB Games, and eBay. So far, I find it cheapest to buy locally, you just need to be quick.