Finding a good condition DSi XL is the hard part. There is a major screen lottery you have to factor in.
Main issues:
1. Yellowing. Don’t buy a used model from Japan, or any other humid region. The screens will more than likely be destroyed.
2. Vignetting. The screens develop vignetting for unknown reasons. It could be from overuse, or no use at all.
3. Dimmer Top Screen. It’s common to find these models where the top screen is significantly dimmer than the bottom. It’s very distracting.
4. Mismatched display color temperatures. In conjunction to mismatched brightness levels, the screens can be opposite color temperatures. Again, not ideal.
5. Dim screens due to age. It’s common to find that both DSi XL screens tend to wear over time. Both screens become noticeably dimmer due to age, but the top tends to go quicker.
To get the best experience, you’re going to want to find a model without these issues. Ideally, you want one with matching neutral color temperatures, if not more warm. The cooler (blueish) temperature screens do look a bit brighter, but they are also much harsher and less pleasing and accurate to me. You also want to find one that is barely used to maximize the brightness.
I have found such a model, and it was open-box condition. If you can find it, the difference is not even close. It’s the best screen on any Nintendo console outside of the Switch OLED. A true enthusiast display.
Your best bet, is to buy one brand new, or open-box condition. Check the screens, and buy junk models online and swap the screens if needed. Ideally the top-screen should have the brightness and color temperature you want. That being the most fragile to these issues, and the hardest to repair.
The only thing I will say, using anything but a good example DSi XL is a direct downgrade to the picture quality. DS Lite and 3DS not excluded, the latter being the worst option.