I have used pellets that are 2 years old with just as good of results as new ones. I keep my powder in my house that is not subject to much humidity or temp change. I have shot deer with 2 triple seven pellets and 3 pellets. I have since gone to loose triple seven, I have had better accuracy with the powder over the pellets.
I don't find triple 7 as dirty as Pyrodex. Triple 7 is very easy to clean up after, but no matter what you use, it is going to leave a residue.