the viper is much stronger. the viper also has a more angular reaction whereas the bullet is more of an arc. the viper can handle much more oil, prlly all rangers of mediums if you have a decent amount of hand. the bullet tends to skid on medium patterns, sometimes on the longer light oil patterns it wont find the needed traction. the bullet is pretty much good for lights or less whereas the viper might hook/burn up on the drier end of the spectrum, but is still useable with speed.

since you already have both i dont see why keeping both would be bad, they shouldnt overlap and will give you a good look on any pattern other than heavy/flood oil patterns.