I don't have the +C so I can only vote on the Tama-go.
I really like it. It's a challenge to get some characters, the figures are super fun, it's cheap enough that if I lose it I can buy another one without having to work for the whole summer, and, frankly, just because something's in color doesn't mean it's better. (Sorry, that sounded really harsh but I didn't know how to say it otherwise)
Now that I've got two figures, I see the actual fun in the Tama-Go. No, I'm not leaning either way, I'm split between the two. There are equal advantages...
TMGC+C
1. It's in colour
2. The games are really cool
3. You can connect with the iD and iD L
Tama-Go
1. The figures are fun
2. Some characters are a challenge to get
3. Training bar and Friendship
...and disadvantages...
TMGC+C
1. PRICY!!! Well, I don't need to list that one, but I got to put it in order to weigh it all up.
2. Gets boring after a few generations
3. Only 12 adult characters in all, in contrast to the Tama-Go's 20 adults!
Tama-Go
1. Figures can be hard to get in some countries, i.e. NZ
2. It is sometimes too hard to get a Perfect Care character
3. Photos of the screen don't turn out well so it's not good for logs
It really depends on these three things:
1. Your budget
2. Your graphics preferences
3. Your ability to care for it a lot
If you have a tight budget but lots of time to care for a Tama, I definitely recommend the Tama-Go.
If you are price flexible and can't pay it a lot of attention, you're better off with the +C.