These are identical cameras with the A520 having one more megapixel.
The A520 will cost you from $90 to $100 more than the A510.
I looked at sample photos of the same subject from both cameras and my eyes show that the extra megapixel costs too much. There is no difference in photo quality and increased detail with the A520 is hardly detectable.
Unless you have a burning desire for a 4 megapixel camera, get the A510.
Since both cameras have manual controls too, they would be great for growing as you get more experience with digital photography.
I am considering these two cameras as a first digital camera. I have read the reviews and would like to know which is better long term. The features are comparable except for the megapixel difference. Which would you recommend for a first camera with room to grow with?