Well first off, we're really dealing with a huge sh*t is situational game now so I can't give any hard and cut rules, but I'll say this to your thoughts.
First off. Atma of Lion versus (other atma) -- Atma of the lion gives 10% triple attack rate which is nice, but it also gives thunder elemental magic attack (useless on thief), and physical damage taken -10% (situationally useful dependent on your activities). Atma of the alpha and the omega on the other hand gives 10% triple attack increase (idential to the lion), 50 attack and -25% hp, so again...what are you doing? I'd take alpha omega long before lion if I was fighting lesser game mobs or farming generic stuff or exp allying, but if I wanted to tank or was fighting high level NMs with really damaging attacks I'd shy away from AO.
Twilight Knife -- I actually never liked the knife alongside our other daggers. I don't know why, but it was never a fav of mine. My thoughts on this are limited due to the fact that I rarely used it.
Gnarled Horn versus SS -- I like this tradeoff. Really I do!! The difference between 20% crit hit rate and 30% crit damage however will boil down to different aspects of our DoT spectrum. SS will increase sneak and trick attack by considerable amounts (30% stronger than before) but the effects on DoT and unstacked WS damage are difficult to measure. The law of diminishing returns applies to critical hit rate as you close in on the 95% cap and we'd still have a 59% DoT crit rate with af3 legs + razed (higher % in WS because crit rate % is boosted on evisceration), but a crit raises pDIF by 1.0 and adds the razed ruins/crit attack bonus traits damage which a non crit doesn't get, so 20% crit rate versus 30% damage is more than just linear math. Sanguine Scythe will have a significant impact on Rudra's Storm/Shark Bite and Mercy Stroke (also Man Stab for Vajra users) whereas gnarled horn is nearly useless in raising those weaponskills damage (50 agi on ta versions is it). The secondary benefits of SS vs GH are counter and 50 agi versus enmity and max hp + 30%, and I like the HP/enmity more than I do the agi/counter. 50 agi is a small bonus to ta damage relative to our total DoT spectrum and the counter is nice but still a fairly low %. 25 eva is nice but I already have a powerful eva swap and have high parry skill now, so I prefer 30% HP to 25% more eva (I also like the enmity increase tbh). I like the SS extra perks more than the GH. SS is a damn awesome tanking atma though, so at the very least when I'm tanking I intend to swap from GH >>> SS.
I could offer sh*t is situational scenarios where you spend more time TA/SA than DoT (HNMs where you're just increasing TH levels while waiting on weakness proc or w/e ...... and this possibly has voidwatch implications but that event is young so I don't know what strategies will be used on higher eschelon bosses or what the role of atma will be after it's figured out). In these cases SS >>>> GH once more.
I've found too many sh*t is situational situations to go by a hard rule of thumb, so I work off basic principles and gauge how effective a setup seems. I judge pros and cons of my choices and use what I like best for the stuff I'm doing. If it's highly effective then I stick with it.
EDIT: I did do some basic numbers on the difference between SS/RR/GH versus GH/RR/AOA and posted them here
. While the considerations of DoT between GH and SS are fuzzy, it's a given that RR/AoA + either SS OR GH is better than a full RR/GH/SS combo. It's not worth sacrificing AoA to have both, so if you want to ago the DD rout then use either GH or SS and keep RR/AoA. Edited, May 12th 2011 7:31am by Melphina