I suppose I'll chime in on this...
First off, the 2DS line, as mentioned, uses inferior hardware to the 3DS line.
If you compare the actual specs side by side, you'll see that nearly everything in the 2DS is a downgrade compared to its 3DS counterpart. I'd imagine, headphones or not, the sound quality of the n3DS beats that of the n2DS (though I have not experienced this to prove it)
BUT one thing that needs to be addressed before you make a decision is this:
Do you WANT to play 3DS games?
This is the first relevant question. If you're not interested in playing 3DS games, then keep your DSi and enjoy it. The DSi is the pinnacle of DS technology.
If you want to play 3DS games, then I'd go for the n3DS simply because of the enhanced hardware. The SD card isn't hard to access. I keep a case on my 3DS and leave the 2 screws loose. When I need to access the card, I just pop off the case, pry off the bottom and take the card out. Takes about 30 seconds, if that. In addition to that, there is the built in wi-fi management that gives you full access to the card (albeit much slower than direct access).
If you plan on running CFW, someone has even extracted the SD Management software and made it so you can put it directly on your Home screen.
TL: DR--If you want to play 3DS games, get a refurb n3DS for $130-$170, depending on where you go. If you don't want 3DS games, keep your DSi.