And why do you think so?  Obviously, you can't foresee all effects of either option (neither can I), so how can you rank them?  (Certainly not by price, iirc fertilizing the oceans would be cheaper.  It's not iron dust, btw, but a ferric salt.)

Planting trees...

Oh, how cute.  It just doesn't help.  To capture CO2, you must also be willing to cut them down and cover them with enough dirt to prevent them from rotting.