As for the 15agi vs +5damage enchant let me point you to this post for probably the most complete calculation you will get here:
http://www.twinkinfo.com/forums/10-...pon-damage-vs-15-agility-cata.html#post190053
As for the shadowfang vs AB question by Falaris.:
Ambush at lvl 19
Ambush
Level 19
60 Energy
Melee Range
Instant
Requires Melee Weapon
Requires Stealth
Ambush the target, causing 190% weapon damage plus (33 * 1.9) to the target (274.93% plus (33 * 2.75) if a dagger is equipped). Must be stealthed and behind the target. Awards 2 combo point.
Now because I am at work I won't take the +agi on shadowfang and the agi on assassin's blade into consideration
Now to calculate lets just say you have 300AP (number doesn't really make a difference) and since all other effect from talents for ambush are percentages it doesn't make a difference because the effect would be applied to both weapons (it would make the difference greater between the two but you can still see which one is better)
300/14=21.4 increased weapon dps.
Lets look at assassin's blade first
Assassin's blade is 15.8dps plus the 21.4 dps from AP for a total of 37.2dps
average weapon damage then is 37.2*1.9=70.68
Now we get the damage part
274.93 weapon damage plus 33*2.75
70.68*2.7493+33*2.75=285.07damage
Voor shadowfang ziet het er als volgt uit
15.8 + 21.4 = 37.2 dps
37.2*2.7 = 100.44 average weapon damage
Now it's 190% weapon damage 33*1.9
So 100.44*1.9+33*1.9 = 253.54 damage
Well as you can see ambush does more damage with Assassin's blade then with Shadowfang and any effect like improved ambush and Master of Subtlety won't change then.