Hi youyang,
1) If talk about output resistance, it can be expressed as Rds=Va/Ids, where Va=Coeff*L. In this connection Rds is independent from inversion level.
2) Because Ids is more sensitive to Vgs the current mirror based on devices operating at weak inversion (sub-threshold) suffers from increased mismatch and noise comparing with strong/moderate inversion realization.
3) Gm can be a matter in constrained design e.g. high frequency operation where Gm/Cg is important.
4) You can simply consider its as an amplifier (current input, current output): mismatch (offset), noise, freq. performance, input/output resistances etc.