First off, I am not going to tell you one way is better than the other. That is up to you to decide after looking at both your goals and resources. I will give you the pros and cons of each.
Only you can determine which route to take, and really most operations use a combination of both. And to fairly compare the two, each operator needs to break down all the costs associated with both and divide it by the number of cows either being A.I.ed or by the number of cows the bull will breed. You also need to determine which bull (A.I. or bull purchased at bull sale) will take your herd closer to your goals. You are not saving money if you are going the wrong direction.