Fiery would give the highest crits and more overall damage. But tbh I don't think you should switch weapons at all, you're wasting 2 gcds. Most of the time I am in a gcd when something procs but even assuming you were just blatantly following someone auto-attacking after switching you have 13.5 seconds of the buff. You get an average of 45.6 damage from fiery with a 3.8 speed weapon in the 3 swings you'll get with the buff. 42 with a 3.5 speed weapon. To be honest though you will almost never be able to get a perfect 13.5 second time frame to benefit from fiery. I would rather not waste two gcds, since I can do 40 damage in 3 seconds usually >.>
Fiery is 240 damage/minute on average with just autoattack. Crusader would give you 200 damage/minute with a 3.5 speed weapon and 163 with a 3.8 speed weapon. (The speed difference allows for an extra swing for smite's within the 15 seconds, thus a much better bonus). Yet Crusader blows fiery out of the water in terms of actual utility, I would much rather have my sader refreshed and keep those 3 seconds