Oh boy, where to begin? Well, I have all three (and they are featured in my
LOG 
) and they each have their good qualities. All three are in Japanese but if you're familiar with Tamagotchis at all, you'll get the gist of it and after playing with a Japanese Tamagotchi for a while, it's like the language barrier isn't even there. I don't know a single word of Japanese yet I (and many others) do just fine with them. I'm even logging about them!
The TMGC+C is simple, takes it back to the basics, and has a lot of cute characters, great animations, and other things (like being able to plant trees out in the garden for instance...the iD doesn't have this feature but the iD L does). I can see it getting boring for a lot of people out there because once you rack up all the GP you can have (99,999) and bought all the items, that's pretty much it. The games are really entertaining. I've had mine for nearly a year now and it still never ceases to amaze me. Once upon a time, I had the same questions as you but I did a lot of research (which is what I recommend for you) and I decided to take the advice of many others and went with getting a TMGC+C first because it's a good first color Tamagotchi to have, so that you can get used to the menu interface (especially since everything is in Japanese). The iD and iD L are pretty much similar to each other when it comes to the menus (they have more sub-menus than the +C, which is why it might be a good idea to get used to the +D first).
I got my iD a couple of weeks ago and most recently, my iD L, so I haven't had the chance to play with them both for a long time yet, but I'll give you my impressions. One of the biggest things that separate the +C from the iD/iD L is that what you see on the +C is what you get. For the iD/iD L, you can access certain download files for wallpapers, games, food, snacks, etc. You can even make your own items. The point of having an iD/iD L is that you download certain things to them in order to make it more "you," customizing it (iD stands for "identity"). To download the items, however, you need access to the iD downloads site (which only people living in Japan can do) and you need a cell phone that is equipped with IR so that when you download the items to your computer, you can use a data cable to transfer it to your phone, and then beam it via IR to your iD/iD L. People have found a way around these two hindrances though because several very generous members who live in Japan download the items off of the site and provide them to members outside of Japan via file hosting sites like Media Share or Deposit Files. You can also buy a compatible phone with IR on Ebay. I got mine + accessories for less than $10 just recently.
Feature-wise, the iD/iD L have more features than the TMGC+C. The iD and iD L are similar but the iD L just has more (characters, a planting seeds feature, a pets feature, the storage of more downloads, etc.). Both the iD and the iD L have two built-in games but as I mentioned above, there's the possibility of downloading more games to your iD/iD L. Even though I love my +C to pieces, there's just so much more you can do with the iD and even more with the iD L. So if I were to choose one, I'd probably go with the iD L. I've had it for less than a week and I'm already in love with it. The only downside to getting an iD L (and any other color Tamagotchi these days, for that matter) is that you might have to pay a hefty price for one. Not only that, all three of them are a little more difficult to come by without having to pay around $80-100 (sometimes more) for one. If you do some searching and know where to look though, you won't have to pay that much. I know I didn't, nowhere near.
I highly recommend that you do your research to see what you might like because we all can tell you many different things but it all comes down to you in the end. Take advantage of the great resources we have here or maybe you might find something while doing a Google search. You might even be like me. When I decided to buy my +C, I never dreamed that I would have the iD and the iD L. I thought the iD series altogether seemed too complicated and I had a ton of questions...I'd have to find a phone, would I be able to figure out how to download things, how to beam the downloads to the Tamagotchi, what in the world are Ta Ma To Mo items, would I have fun with them even if I didn't download things (the answer would have been yes, though having the downloads are nice), etc.
HERE is a great guide on the +C.
HERE is a menu and stats screen for the iD L, just so you have an idea of what it's generally like.
HERE is one for the iD. We don't have many threads here for downloads unfortunately but you can find the ones for the iD
HERE (scroll down to the topic that says "Download files from Tama-ID.com;" you can also find a list of compatible cell phones in that list of topics). Since the iD L just came out (in March), we're only just beginning to have access to some downloads for the iD L
HERE, again, thanks to very generous members who live in Japan.
I will be here forever if I were to tell you everything there is to know about these three so again, I recommend checking things out, do a bit of searching, read up on logs, and ask specific questions. Good luck!
