Nokia lumia 610 or iPhone 3GS?

Wich should I get?

Get the iPhone. In a few years it will be worth a lot.

I'd instead recommend the brand new Nokia Lumia 520 - which should be the same price as the Nokia Lumia 610 and the iPhone 3GS.

It has Facebook, Twitter, Xbox, Office and Skype built in. There's HERE Maps for completely free, voice guided, turn by turn navigation; and Nokia Music for completely free and unlimited music mixes. There's also more apps available to you in the Windows Phone Store.

Over the Lumia 610, the Lumia 520 has wider app selection, more security features, new functionalities (like Bluetooth File Transfer) and better performance.
Over the iPhone 3GS, the Lumia 520 has a larger screen, a better camera, expandable memory.