Android phone or iPhone 3?

I want to get a decent phone, that is going to last me for at least two years. I have narrowed down my choices to the iPhone 3 and an Android phone. I would probably get the LG Optimus S if I got an Android. Since I have TMobile, and want to stick with them, I can only get an unlocked iPhone 3, the iPhone 4 has a different SIM Card size. I do not have an Internet or Data plan of any sorts, but I can connect to Wi-Fi with both phones. Both phones would be similar in price for me to get, but I'm not sure which one I should buy. Please leave reasoning with your answer.