It's hard to say these days, but I ended up voting for DS Lite over the DSi. However, both have their pros and cons.
Pros for DS/Lite: * The GBA slot. Not only has my DS Lite replaced my GBA SP for most everything, I've heavily abused the GBA-DS link-up feature and really appreciate it. As long as I play 3rd gen Pokemon games - which is actually still pretty frequently - I'll be using the Pal Park.
* Better battery life. This has become increasingly noticeable the more I use my DSi.
Pros for DSi: * Downloadable content. I love the games I've downloaded so far, and I'm sure many more interesting ones will pop up over time. In the end, it's just fun having games saved directly onto the handheld.
* SD card usage. I didn't realize how great having that addition would be until actually purchasing an SD card. Now I use it between my DSi, Wii, and laptop. It's awesome.
* Better wireless, external sound, slightly bigger screen, etc. All of the spec improvements I appreciate, even if they are minor in some cases. It just one-ups the DS Lite and makes it better to play on. I've designated my DSi for Pokemon Platinum, and while I didn't notice the larger screens at first - I do now.
Not gonna bother with cons. The pros lists kinda say all. Either way, the GBA slot is still a very big feature for me, and because of it I could never stop using my DS Lite. Still, I'm happy with my purchase of a DSi and am really enjoying owning it ^^
*Lolz at poor little ignored PSP. I actually do own one, it's just not even a contender for me vs DS...*