Keep the DSi if you still wanna play old DS games.
semi-co-sign
DS games on the the 3DS look worse than I expected them too. It's pretty blurry and butty, plus the DSi had larger screens. If you don't care about image quality, the games play and perform like they did on the DS. But yeah, the DSi is still the best way to play DS games.
also, yeah, the AR stuff is really, really neat. One of the games you unlock (stupid that you unlock mini-games, btw) is like a golf game where you physically move the system around a structure to aim the cue. Another has you fishing on your desk with the DS as your fishing rod. Even the simple shooting game requires you to move around to hit specific targets.
I don't know if you're supposed to play those in 3D but I had to turn it off because I was constantly moving the unit around. Hopefully Pilotwings shows off the 3D more because just based on the various features in the menu, it's not doing much for me. The 'sweet spot' is smaller than I thought and I didn't realize you had to hold it far away from your face. The 3DS is telling me I've been incorrectly playing portable games all these years, so maybe that's a good thing. I was holding older systems too close to my face and was getting face cancer or something.
And fuck the mii maker. I did it three times and it makes a Mii version of Jason Schwartzman. I'm getting a nosejob on wednesday.