Only thing Pierce has over him is a ring, which he got as a member of a team much better than any team Melo has ever played for.
Pierce career stats - 21.3 ppg 5.9 rpg 3.8 apt 44.7% fg 80.7% ft 37% 3p
Melo career stats - 25.3 ppg 6.5 rpg 3.1 apg 45.5% fg 81% ft 34.5% 3p
Both are clutch, Pierce has had many more opportunities in big playoff games, if Phil can put a real team around Melo or Melo eventually leaves for a true contender and can add some big time clutch playoff moments Melo should go down as the better player by a pretty decent margin.
For the record I'm not a Melo fan, I'm indifferent towards him