What would/do you use, and why?

I personally prefer when the damage goes out more frequently. Specially when i wanna move thru packs faster and deal with annoying small enemies before they "run back" for example.

For longer bosses it dont matter much for me id be ok with either.

I'm leaning towards the 2,5 sec for exactly the same reason...

4, it pops early when you do enough dmg to kill the mob so I don't the downside of having max time. the plus side is up to 56% extra dmg at 4 seconds for dmg res/more health mobs and bosses

There will be a lot of people claiming that 4 is better, but if you play T100 you'll notice that the 2 sec pop is much better because the damage chains like crazy.
Only on single target 4 seconds is better.