Gee that's quite a tough one! I think both Greene and Smith are not really worth selecting because I think neither will make the top 6-10 in their respective positions. Out of the two though, I think Greene has a better chance of hitting a reasonable average. At worst he could be traded out around the Byes.
Crows have a tough draw to start so I think Smith will 'trap' his owners again. (Reference Beg2Differ's 'Maddinson Trap' comment from the BBL fantasy competition). Much like Maddinson, Smith will have one fantastic score to show his potential followed by a run of garbage scores to show why he's priced at what he is. Smith will most likely put in one great NAB score to suck everyone in again (I hope he does!).
Out of Montagna and Boyd, Boyd is the better pick due to his position. If he can average 100+ in the defence, that's much more valuable than a 100+ in the forwards.
So Boyd + Greene for me!