The size and shape are the same. They both run on AA batteries. They both have bright 3-inch LCDs on back and a 6x zoom lens on front with optical image stabilization. Yet, the change from gray to black makes the camera look like it performs better. (Sadly, they both have fairly mixed performance.)
So if you don't care about the body color, why go with the A2100 for $50 more? Well it has Canon's Digic 4 processor for better face detection among other things, a 12-megapixel resolution compared to the A2000's 10 megapixels, and the A2100 has Canon's Smart Auto scene recognition.
However, both cameras take excellent photos regardless. (At least below ISO 200, that is.) It's just that you can do a little more cropping with the extra resolution on the A2100.
And it is black.