I guess more specifically, is it worth it for DS-10 Plus?
This would be my first experience with any kind of synthesizer, so I don't know if the improved functionality with the DSi would be immediately useful to me. But, in the long run, should I just pony up the extra forty for a DSi?

I would say no. The camera and other similar features are something you can find on other electronic devices in a better form while the DSi shop doesn't carry too many gems, especially for someone who has yet to experience the full library of DS games.

Yes, , I have the original software and it feels much better due to the improved sound and larger screen size. I am getting Plus also,  You could make animations in Flipnote Studio (free download) and them make some music to accompany it.
 

I'm definitely getting plus, but i didn't get my DSi for it. What i will say though is that DS-10 plus fixes pretty much everything i complain about with DS-10 vanilla, and if you are serious about making electronic music on the go with a good softsynth emu and a decent gui that takes itself seriously, i'd get a DSi for this. DS-10 has barely left my DS since i imported it way back. It's that good.