Some legendaries are better than others for DPS. Some legendaries are better than others for survivability. Some legendaries are better than others for utility. But which should you use?
First, I determine if there are ‘hard’ features I need/want. For example, if I want to maximize my self healing and survivability, I might go with the neck and the helm. Prydaz (neck) offers a very nice damage absorption shield which amounts to a lot of effective healing (think of it as healing the damage you just took immediately). The helm provides a great heal when you Feign Death. So if I am in LFR, or on a progression boss where DPS isn’t a priority, I might choose those.
In farming Argus, I use a ‘Leech’ gear set that I cobbled together. These aren’t always the highest iLvL gear in my bags, but I prioritize Leech on as many pieces as possible. Then I will pick legendaries from the pool of gear that I will replace that doesn’t have leech. So I might use lesser value legendaries simply because the gear it is replacing doesn’t have leech.
For DPS however, it can get more confusing. Many people have asked me which they should use, so let me offer the method by which I determine which legendaries I use at any given time.
First I determine what non-legendary gear I have that is best to wear and put that on. Than I figure out which legendaries are best for that set of gear. Let me give an example of the logic:
Let’s assume (for simplicity) that all of our non-legendary gear pieces can have a value of 1-5, 1 being the worst and 5 being the best. Let’s also assume that the legendary pieces have values from 6-10 (6 being worst, 10 being best).
Simply putting on a legendary with a value of 10 might seem to make the most sense, but that isn’t necessarily true. Let’s use 6 pieces of gear that have values of 1,1,2,3,4,5 respectively. And we have legendaries to repalce that gear with values of 10,8,7,6,9,10 respectively.
Now I compare the relative upgrades: 1 to 10, 1 to 7, 2 to 8, 3 to 6, 4 to 9 and 5 to 10. This provides an upgrade difference of 9, 7, 5, 3, 5, 5.
Looking at those relative upgrade values, I would probably choose to upgrade the first two slots, because I am getting the most value doing that. Upgrading from a 1 to an 8 is more valuable than upgrading from a 5 to a 10, even those the 10 is ‘better’ than the 8.
I really hope that makes sense. Let me know if you have questions 
Gween