The 25 grains will change how your shaft bends as the string accelerates it foward. If you have a very stiff arrow shaft then adding weight to the front of the shaft will essentially weaken the spine of the arrow. This could result in a better tune and tighter groups.

Download Eastons tuning guide and read it. It has far too much information for me to post here. It will explain how things like tip weight can effect the flight of your arrows.

As far as which one you need it would depend on your draw weight, draw length, arrow length, how much energy the bow stores (how quickly it accelerates the arrow) & spine of the shaft.