I just got a used DSi XL for under $100 from GoHastings with my 40% off code. I already had a DSL and 3DS, so I really didn't need a DSi, but I really wanted the larger screen of the DSi XL and I don't expect Nintendo to be coming out with a 3DS XL anytime soon.
My first impression of the DSi XL was how big the screen was, but then I noticed how pixelated the screen looks since it is really just a DSL with bigger pixels (at least that's what it looks like). It made me realize just how much nicer the 3DS screens are (MUCH nicer)
But having one of each , I'd recommend the following:
1) If you have any GBA games (or want to buy some), you need to get the DSL. But if you don't already have a bunch of GBA games, there are so many good DS games to play I'm not sure why you'd want to spend time playing GBA games.
2) If you have one of the GoHastings 40% off codes, you might still be able to pick up a used DSi XL for under $100. If so, that's the way to go unless "in our pocket" portability is critical since the bigger size makes it less pocket friendly.
3) If you can't get a good deal on the DSi XL (or don't want one), then get a DSL since I can't see how the benefits of the DSi over the DSL justify the higher price...and with the DSL you retain the option of using the GBA slot for GBA games or some DS games that use the GBA slot (such as the Guitar Hero controller, the rumble pack and the paddle controller). I'd only get the DSi if you can get a GREAT deal on it.
Eventually you'll want a 3DS, so the less you spend on a DSL/DSi XL the more you'll have leftover to buy a 3DS when the prices become more reasonable.
I also wouldn't be afraid to buy a used unit if you can check it out first or the place you buy it from has a good return policy. Fortunately the DSi XL I got from GoHastings was in excellent condition (of course, they screwed up the other part of my order by shipping a Fat PS2 instead of the Slim PS2 I ordered
).