You will very rarely have 2 crusader procs up at the same time on a boss.
The whole point of Crusader is to be able to pool your energy around it and pop all your CDs once you get a proc during a close to ideal situation (aka you will have more or less 15secs of uptime on the boss).
Simple reasoning to compare Crusader vs 15 agi, both for MH & OH :
- Crusader is based on 1 PPM iirc (feel free to correct me though). Meaning you can expect 1 proc to occur every minute on the paper.
- Most bosses (it's around 80% of current bosses for me) will die way before the 1 min mark. The chances you don't get a proc on a boss fight are therefore pretty high.
Example : Some bosses in MC will die in around 15 secs, meaning you can't afford to wait for a Crusader proc. You need to use your CDs as soon as the boss is engaged. In 15 secs the chances you get a Crusader proc are garbage tbh. You're playing lottery with your dps at this point.
Conclusion : Even on MH, 15 agi would be more reliable and consistent. Crusader would be preferred if you're getting long fights (above 1 minute at least) where you get a melee uptime for most of it and you can afford to save your CDs when a proc happens.
This whole reasoning is different for trash mobs. Especially if you have little to no melee downtime when switching from a trash mob and pack to another.
I'd say it depends on
your playstyle and
how fast stuff is dying in your raids (
go for the enchant with the glow you prefer 
)
In short : Crusader for frustrating casino dps, 15 agi for consistency.