For me, MWC is only replaced when I have the combo CoC hc + WoU hc. So, the answer is, sim it yourself and see. (Probably, you will get mwc + coc normal as the best pre-heroic trinket combo)
My char started simming higher when i replaced MWC with WOU N and having CoC N. There are results:
3.31.7:
- With CoC N and WoU N - 44342 DPS
- With CoC N and MWC - 43395 DPS
- With WOU N and MWC - 43286 DPS
0.31.10:
-With CoC N and WOU N - 44180 DPS
-With CoC N and MWC - 43279 DPS
-Woth WOU N and MWC - 43133 DPS
It denepnds on your current gear and stats.