Sony Ericsson Z520 / Z525

IMO, the 520 blows the 500 out of the water in about every aspect (better internal screen, it has Bluetooth, I like the smaller size--thinner, just better looking, keypad is better, more memory, reduces background noise better, I like the blue background illumination of the keys instead of orange.

The only things I like better about the 500 are that mp3 ringtones I made sounded better on the 500 and the external screen is easier to read b/c it's bigger.

I personally like the 520 better because it is smaller and it also has a few more features than the 500. The only thing that I have noticed that I don't like as well is the speaker phone, it is louder on the 500 than the 520.