Metal vs. Wood. Both would be strong enough to hold your saddles. One problem with metal is that they can get very hot inside if you live in a warm sunny climate. One recommendation I will make is that if it gets very humid where you live invest in a dehumidifier. Moisture in the air will cause mold to grow and will "rot" and ruin your equipment.

Depending on how many saddles you have and how much tack you need to store you could be able to get a good deal on one of those small storage sheds and then just buy the storage equipment you need (saddle racks, etc.). A cheap way to store equipment is to hang used horseshoes on the wall to hang bridles, ropes, etc.