I only take US dollars but I'm in the US and most of my sales are in the US. As for the $100 or €100, I'd take the euros given the chose. That's like $135, who wouldn't rather have more dollars?
I'm about to setup an e-commerce site and this is something I've been pondering. Given the weakness of the US$ and my fear that it's still got some way to fall, I'd prefer to take payment in euros.
I would use Indian Rupees as the currency medium while dealing with Indian customers, living in India. But for those who are from other countries irrespectively, I would certainly use US dollar as the only option. Its widely recognised.
US dollar even though I'm located in Canada and so is the company itself. The reason being most customers are in the US or they're overseas who also rather deal with the conversion to USD from their currency opposed to this mysterous CAD.