I've been doing a lot of research on DSLR's as i'm looking to buy one myself. The D3100 is a good entry level camera but it has one major flaw, it doesn't have a focus-motor built into the camera.
This means that once you've learned some basic skills and want to buy lens you will have to buy much more expensive lens (AF-S) compared to lets say a Canon 550d. AF-S lens are more expensive because they have a built in focus-motor, if you buy regular lens they won't be able to auto focus and you will have to do it manually.
Whereas with the Canon 550d the focus-motor is built into the body. So the Canon 550d might be a little more expensive initially but it will save you money in the long run as lens will be cheaper compared to the Nikon D3100.
Canon 550d is available for 67-68k with the kit lens (18-55mm)
Nikon D3100 is around 53k with the kit lens (18-55mm)
These are karachi prices, they're slightly higher in islamabad.
So between these two DSLR's i would recommend the Canon 550d. Besides having cheaper lens it also has a better feature set. Hope this helps.