Merit is generally the superior of the two. Seniority should only really be preferred if:
The performance of the task at hand is more dependent on pre-established connections and the like - ie a very knowledgeable and charismatic but new diplomat might actually be a worse choice for a given job than a rather boring and mid-tier diplomat who nonetheless has a lot of long-standing relationships in the area.
There is an issue of systemic rules in an institution where merit is not always the top concern, or not easily and clearly discerned - such as legislative committee appointments. “The best person is the most qualified for the top job” is a nice thought, but raises questions of “How do you figure out who is the best person?” and “The people didn’t send the best person, they sent this fellow to represent their interests.”
Merit is generally the superior of the two. Seniority should only really be preferred if:
The performance of the task at hand is more dependent on pre-established connections and the like - ie a very knowledgeable and charismatic but new diplomat might actually be a worse choice for a given job than a rather boring and mid-tier diplomat who nonetheless has a lot of long-standing relationships in the area.
There is an issue of systemic rules in an institution where merit is not always the top concern, or not easily and clearly discerned - such as legislative committee appointments. “The best person is the most qualified for the top job” is a nice thought, but raises questions of “How do you figure out who is the best person?” and “The people didn’t send the best person, they sent this fellow to represent their interests.”