The id is a little more expensive but that's only because it's english the TMGC+C is japanese.
@memetchirocks2122321 actually, the iD is in Japanese as well, as far as I know.
@Zzgs The iD allows one to download files from a Japanese cell phone (the phone downloads the files from a mobile website, and the files are sent to the iD). On
Tama-Zone (Look under "Japanese-language Tamagotchis"), some users have figured out how to send the iD files from a computer to an IR phone and from there to an iD, or even create their own files to send to an iD (That's pretty technical, though, and unofficial).
You can also dress your Tama up a bit, and there is a "TAMATOMO" function where you earn different Japanese characters (as in the symbols that make up the language) by discovering your adult Tama's favorites and such. The ability to plant trees outside from the Color I think has been removed (I don't have any of the color Tamas).
The Color, I think, is probably simpler in its list of activities, but tends to go for less than iDs on eBay, since they are an older kind of Tama.
If you want to save some money, and are not really willing to try to download items or create items, then you may wish to get a Color instead (The Hexagontchis are even less expensive on eBay, though there is quite a bit of Japanese language in sections that could make playing it difficult for foreign users without a guide).
Sorry for dragging on, but I hope I helped!
EDIT: Looks like I was beat to the "both are Japanese" thing. It's fine, though.