What are the intended roles for the carbine ? The idea is to fine the answers based on your needs.
Examples :
3 gun competion & want to win regional matches.
LEO hard training and hard use carbine .
Civilian fun gun used for some training , some hunting , and kept handy for possible home defense.
ETA:
There are some very good threads about which triggers are good or bad for specific applications on M4carbine.net . I personally would run screaming from a room full of arfcom yutes if they even started to discuss this topic.