A difference between aciclovir and penciclovir is that the active triphosphate form of penciclovir persists within the cell for a much longer time than the activated form of aciclovir, so the concentration within the cell of penciclovir will be higher given equivalent cellular doses.
One key distinction between aciclovir and penciclovir is that the active form of penciclovir stays inside cells longer than the active form of aciclovir. This means that even with the same amount of medication, there will be a higher concentration of penciclovir within the cell.