both wok, but if your upgraded, use .25g without question. They may move slightly slower, but they travel further, are not effected as much by wind or leaves, travel flatter. Rember, its 25% more mass then the .20g
LUTNIT already outlined some reasons, so Im not gonna hit on them, but there are several good threads discussing this.
Rember for GBBs, they are designed for .25g or heavier.
I use .25g
and only .25g
If I run out of .25g and have a bag of .20g sitting beside me, Ill run back to base for more .25g