So Tama-Go:
it's inexpensive, has a black-and-white display, and is almost always in English. [except in one video I saw of a Portuguese one. But the Portuguese one has anime characters on the package, and I'm sure it would be specified as Portuguese. Find the thread I made about the Portuguese one
here. (note: I was freaked about the prospect of a Portuguese Tama-Go at the time)] To improve the Tama-Go, you have to purchase figures, which have games and shops on them.
Tamagotchi iD L:
It's fairly expensive for the Japanese one (around $60) and very expensive for the English one (around $100 and somewhat hard to find). It has a color display, and many features such as downloads via infrared connection, etc. It is a very amazing Tamagotchi, but expensive. However you can improve the Tamagotchi for free using infrared downloads (if you've got an infrared cell phone or USB to IrDA dongle).
If you're speaking of the Harrods English iD (or maybe iD L) offer, my guess is that they will be in English. However, they'll be very expensive, and I would suggest waiting until other people have bought them before you try it yourself.
To buy a Tama-Go, check out online stores like Amazon or eBay, they're your best bet for a good price. If you have a retail store in your area that sells Tama-Gos, go for it!
The iD L you will have to purchase online, unless you take up the Harrods offer (find the thread in Comments and Rumors on tamatalk). They'll be expensive, but well worth it. On tbay, you can find English iD Ls, but also Japanese iD Ls. You can also find Japanese iD Ls on CDJapan (that's where I got mine) for relatively inexpensive.
Tama-Go vs iD L main differences: price, black and white vs. color display, different characters and games, iD L has download ability.
So if you want a cute, easy tama for cheap, go for Tama-Go
If you want an amazing, beautiful tama for a much higher price, go for Japanese iD L, or English iD L if you want it more expensive.
Edit: whoops, Midorime and I posted at the same time with the same basic points.