That depends on your class. For a hunter the Backwood = 1% Crit, 42 AP from agi, 135 mana, 130 hp and a bit dodge and crit from the agi. This is just 200hp, 40 AP (same as 20 agi) and a crit. Difference: Some mana, some dogde and som crit better with the backwood but 70 hp more with Tyranni. SO for a hunter i would pick the Backwood anyday. An Enchantment shaman on the other hand is different. The backwood gives 1% Crit+1% crit for agi (20agi=1crit), no AP at all, less hp and some mana. I personally prefer a high AP scince the damage becomes more constant and less bursty, which is great for PvE and thanks to WF we get a lot extra attacks so the crits are gonna come anyway.