We will take a make believe trinket and compare.
Trinket 1: Proc Trinket : Procs 4000 agility on hit for 20 seconds, 90 second internal cooldown.
Trinket 2: On Use Trinket: Grants 4000 agility on use for 20 seconds, 90 second cooldown.
Now looking at them if the person uses the on use one ASAP when it is up and it is off the GCD so it does not waste time there is no difference between the two. Both will give you 4000 agility every 90 seconds. Simple enough right?
Not really all that simple. There is a lot more to consider and the changes to hunters will make even those people that played them know how having more keys to press makes a huge difference. So many people have had problems adjusting to the changes made to hunters because they were given 4 (or 5) new buttons to hit. Having more to hit means more chances to make bad decisions and press things in the wrong order.
The more buttons a class has the less trinket 2 looks good. If you are already worried about hitting 12 different things adding a 13th is not going to help matters any. You would be better off having things go off on a timer on its own with nothing needed as far as input goes from your end. With the on proc trinket the agility will always be there when it is time for it to be there. You never need to worry about it ever. It is a set it and forget it thing. You can't ever make a mistake and not hit it because it will hit itself.
So that clearly means that trinket 1 is a better trinket. For that case at least.
When you are raiding and there is a need for some burst damage having trinket 1 could leave you out in the wind. Will you have that 4000 agility when burst time comes? Who knows really. With trinket two you can save it for the moment you need it. During that phase where you need extra damage to burn down an add or during the hero/lust phase or during a time the boss takes extra damage. Just use your on use item and you are ready to burst when you need the burst 100% of the time. Never will you have to hope your trinket procs when you need it to because you control when and were you get that extra agility.
So that clearly means that trinket 2 is a better trinket. For that case at least.
So while there are some cases where one might be better than another, like a fight that has no particular type of burn phase would lead to the proc one being better as there would be no chance of missing using it, it really comes down to which you prefer.
Hard core raiders getting every last ounce out of what they do will have both and use both in which situation it is needed. I know I always have both even if I am not hard core, just in case there is a fight where I need my burst available when I call on it. But it is not really required as I am not hard core, so that leaves me to ask what about the rest of us. We should just go with what we like.
What do you like?
Last expansion I liked on use. I used to stack things so I used it at the right moment for me. While it would not have as much of an uptime as a proc trinket for example because I held it back and used it at times where I found it would be best to use it for burst, I made up for not having as much of an uptime as the proc one by stacking it.
Now, I am on the other side of the fence. I like the passive nature of things that take care of themselves being I have so many new things to worry about as a hunter as is right now. Perhaps my opinion will change as I get more used to all our new abilities, as it seems I am as time goes by, but for now I like proc based trinkets over cooldown based ones.
Unless you are hard core what you would use is based on opinion really. Even more so if they would end up with the same net result.
So while most people will just look at a best in slot list and choose whatever they see is best in slot others will go with what they like and what they feel more comfortable with.
If there is one mistake I have had to teach people not to make as a raid leader over my time doing it about trinkets, it is that on use trinkets are worthless if you do not use them. So often I would see people with on use items on and then looking at the log notice that they never used it once. When talking to them they said it was best in slot. Yes, it is, but only if you actually use it.
For the majority of players I think proc based trinkets are better. There is no need for anyone that is not planning when you use it for for its best use, before the fight even starts, to ever have an on use trinket on even if some guide calls it best in slot.
Which do you prefer?