Reasons to buy a Nikon D3100
-1080p Video at 24 Frames Per Second (Continuous Focusing)
-14.2 Megapixels
-Newer technology sensor
-Guide mode/ The camera sets your settings for you per the effect you want. You will learn so much if your a novice to DSLR's.
-Compact frame
-Cheaper
-Larger Screen
-Newer technology
Reasons to buy a Nikon D5000
-Bracketing, Makes taking HDR Photography much easier
-Larger Frame
-Flip out screen (this is a nice feature)
-Larger Sensor and slightly better image quality
-Wireless remote Option
-Can take 4 Pictures per second vs 3 on the the D3100
Both cameras are excellent.