In Chile, which prepay sim card is best for iPhone 4 (ie has data)?

I'm visiting in a few days and would like 3G access. Can I get this as part of a prepay/prepago plan? I don't mind cutting up the sim to fit the iPhone 4.
Whilst I'm asking, is it possible to get it to work economically in Argentina too or would I be better to buy a sim card there too?
Many thanks
 
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