I recently bought Nokia 3610 and 2100. Both cost approximately the same though 3610 is expensive by a couple of hundred Rupees.
Both are claimed to be entry level phones by Nokia and boast of approximately the same features. Though the menu order is different. Some features are found in 3610 which are absent in 2100 and vice versa so its a tradeoff between which feature you find more useful. For example: 3610 has the facility of adding voice tags to call people by using voice dial. This feature is not there in 2100. However 2100 has keypad locking with password which 3610 does not have.
3610 has WAP and 2100 doesnt. 3610 also has more games compared to 2100. However 2100 has newer and better ringtones and screensavers.
The backlight and keypad lights of 3610 are much brighter than those of 2100. It depends on personal preference which one finds more appealing.
I found the Off button on 2100 much more harder to press.
Both phones are almost the same size though 2100 is slightly thinner. It is difficult to insert and remove the sim cards for both. 3610 has the additional problem of the back cover being harder to remove (in my humble opinion)
3610 is the older phone by about a year, so I would guess that it would depreciate in value pretty fast. On the flip side since it is older and more widely used it is probably more reliable since there are no reports on the reliability of 2100 yet.
Bottom line is that both are almost the same and finally it boils down to personal choice of which attracts you more.