3100 dont has a camera. Between 2600 or 3100 means 3100 of cource. 2600 is a very basic model.
In case u want a camera then go for phillips C155 (i am not sure abt the number).
J200i of SE also is a good phone under 5k. It got awesome speakers and a large display.
LG1600 is another option. It got 65k colors, voice recording, GPRS/MMS and it's less than 5k