As grains go up velocity goes down   which is more important and why would you select one over the other ?    preferences?

Check the ammo-oracle for great info.

velocity decreases as bullet weight increases only if all other factors are equal.

Energy = mass x velocity squared, so you can increase energy by increasing weight, but you can increase energy much more by increasing velocity.  This explanation simplifies thing a little too much, as bullet performance is dependant on many, many factors, but you get the idea.

Check the oracle, you will be amazed at the quality of the information.