Which wireless USB key is the best to connect to the internet?

Orange has less negative feedback. So I'll choose orange.

If you're planning to use internet frequently, for research, to watch films online and listen to music, then I'd recommend you to opt for ADSL.

Dongle is best if you'd check and send your mail occasionally.. In other words dongle is best for businessmen, who always have to move around for meetings and so on..

For orange I only know about the cost.The USB key is about Rs2000 and you can get other associated cost details here in orange

I don't much know about Emtel or Orange but I would not recommend MTML for several reasons:
After the first few week the relatively high speed [compared to adsl] can drop to around 25 Kb/s [downloads]
When you receive your bills the tariffs seem to differ a lot from what you are told on subscription. There are charges they omit to tell you about.
There were periods when I did not get any connection. Which doesn't mean I did not have to pay the charges.
They can forget to send your bills. You will of course be charged interest on late payments (on the late bills)

For a moderate [occasional?] user of the dongle those failures in their service might go unnoticed but if you need to stay online for long periods or have to make a lot of upload/download make sure you have a fat wallet and another internet connection to fall back on.

I am a bit confused as I was told that Orange usage goes down very quickly. Example if you buy a prepay card, the card get used up quicker than Emtel. (I have only heard...but never verified)

Has anyone ever made a comparison?

I only need to use it occasionally at work as I have my normal MyT at home.

Thanks