They are both primarily DPS stats. Agility grants some armor, crit and dodge. Strength increases attack power so will increase the amount of your shield. Agility however wins out between the two as a druid tanking stat you also gain dodge from it. However, in slots where strength may be found (back, rings, and neck) those pieces with strength will have other defensive stats because they are tuned for tanks. In those slots, the tanking items will be better than the DPS items unless it has an unusable stat such as block or parry.
Strength is not better for a druid tank. It may increase your DPS while tanking, but as a tank your primary concern is survivability. Agility grants more survivability. In a direct comparison of these two stats, Agility will always be better for a druid tank. However the comparison is never so black and white.
You have an agility item, then that means all other stats on it are DPS stats (such as hit, haste, crit, or expertise). Tanking items which have higher stamina sill have strength on them as well as tanking stats (dodge, block, parry, defense, hit, or expertise). Provided the tanking stats aren't worthless, these items grant higher survivability because they will have stats focused on tanking. 100 dodge rating is better for a druid tank than 100 crit rating. Because of the other stats on these items, it will overtake the value of the lost agility (in part because of the increased stamina on tanking items).
"In anything, if you want to go from just a beginner to a pro, you need a montage." /w TankSpot WTB Montage for Raiders.