Dave:

What caliber is the rifle?

Generally a lot of people as a rule use 10% less 3Fg powder compared to 2Fg. That is because 3Fg being finer grained burns faster generating more pressure in the breech. The upside of that is that there is generally less fouling using 3Fg.

Since smaller caliber barrels have greater tensile strength, they can better handle the stronger pressure created by the ignition of 3Fg powder.

The different burn rate between the two powder sizes will also affect accuracy. I find it best to work up the load in 5 grain increments until you find the best accuracy. If you have both grain sizes, you can then compare your rifle's perfect load with each grain size and then stick with what works best.

That is the great thing about this passion of ours is that you can work up your loads on the fly while at the range.

Good luck!