Self Certifying vs Senior Certifying
According to Human factors and behavior analysis, I could say that Senior Certifying would win in the Safety grounds.
At normal conditions either of the ways would be considered efficient in safety standards. But at abnormal conditions, like for example Mental tensions, Health problems, bad distractions, alcohol consumption, bad work place ethics, in these kind of situations : A single person doing the work and certifying the work himself, could not be considered safe. Where as, another extra guy / senior remonitoring the work performed, could increase the safety levels.
At normal conditions, and people working under good environment, could not find the difference between the FAA and EASA system of Releasing the Work.
While under the rarely happening Abnormal conditions, unwanted issues would be arising deviating everyone's minds from their normal thinking process.
For example, prominent airlines had been penalised due to inefficient/malpractised maintenance practices, in the year 2008, some licensed technicians supposedly have released the aircraft without actually rectifying the defects, further to say - in an efficient reputated facility and considered controlled environment.
To maintain Higher Safety standards, in comparision, having an upper eye with in an organisation, would always helps, in all conditions and environments.