Ask yourself a couple of questions to see which one you want:
1) Do you want to play (or have the option later on of playing) the older Game Boy Advance games?
- this is the one main advantage that the DS has over the DSi
- You can find many GBA games at really good prices (there are literally
hundreds of used GBA games on
Amazon for under 2 dollars)
- This basically increases the amount of games that the DS can play that
the DSi cannot
2) Do you want internet access?
- the DSi does give you the ability to go on-line, whereas the DS doesn't
- Keep in mind though that you have to have access to a wireless router
(one that's not locked - usually the one used the most is the one
that you would have at home for your PC)
- The internet does not work great on the DSi. It's pretty slow, and not
that easy to navigate around with. We've only used it a couple of
times.
- It does offer a very safe parental locking feature. Without your 4 digit
password, you can't access the internet at all.
3) Do you want a camera? This is one of the biggest advantages of the DSi.
- My son has spent many hours taking pictures, and distorting them
since we got the DSi. It's has very easy to use photo features.
- Some of the games you can download on the DSi take advantage of
this photo feature.
- There are some new games being released that will only be usuable on
the DSi which also take advantage of the built-in camera. If they sell
well, I can imagine many more in the future, thus limiting the DS as to
which games it can play or can't play
4) Do you want to be able to download games (at pretty good prices) from
the DSi store. Right now there aren't a lot of good choices to download
but hopefully as time goes on, there will be many great games that you
can only get for the DSi, that the DS won't be able to play.
- Wiiware for the Wii has shown that it is possible to see some great
(though smaller) games that many developers like to try out. The same
should happen for the DSi
5) Do you want a bigger screen and better sound? the DSi has a screen
that is a bit bigger than the DS, as well as better sound quality (not a
huge difference, but it is a little bit better.)
6) Is price a major consideration?
- The DS is definitely cheaper (and we'll probably see more sales of it
as the holidays approach in order to sell out all remaining systems).
- this is the other major advantage of the DS over the DSi
We have both. My son had a DS for quite a while, but when the DSi came out he really wanted that, so we got that for his birthday (and I inherited his DS

). He does seem to enjoy his DSi more than he did his DS (mostly because of the camera, but also the bigger screen).
Right now both systems are almost 100% compatible (with only a few DSi games using the camera that the DS can't play). In the future, we'll see what happens.
BTW, as to the new DSi XL, don't worry about that. It was made specifically for the a large portion of the Japanese audience (the older generation) who complained about not being able to see the screen well enough. The thing is really big, but besides that nothing else was really changed.
There will be a newer DS coming out in a couple of years (possibly being announced next year at the E3 gaming conference), but it will almost definitely be able to play all the current DS games. (They are even talking about it being able to download all the GBA games as well - but we'll have to wait and see on that).