Strengths of Mirroring:
- Keeps forum community informed of mailing list happenings
Provides different communication method with more "features"
Weaknesses of Mirroring:
- May cause fragmentation of information
Strengths of Single Direction:
- Simpler to implement, less moving parts
Weaknesses of Single Direction:
- Makes fragmentation more likely since information will not flow to users who only subscribe to mailing list
Archiving efforts may only grab the mailing list
Strengths of bidirectional:
- Keeps two information sources more tightly connected
Weaknesses of bidirectional:
- May be another avenue for spam to reach the mailing list
Another thing to maintain, more moving parts
I feel that implementing either way will be worth it if a clean solution is reached but single direction is definitely worth implementing. If not solely for the fact that more copies of mailing list data is better from a retention standpoint.