Not my most stellar paint job, but more than adequate to the task.
The first and most obvious difference is the bottles, so I'll start there. I don't have any particular love for the Citadel ones, but the P3 bottles are terrible. Half of the lids broke off during this experiment. It was incredibly frustrating and led to a few spills.
Surprisingly, I was not impressed with the dropper bottles either. I really thought they were going to be my all-stars of packaging. But in reality, they were frustrating to mix. Both the Vallejo and Reaper ones had problems; they took an absurd amount of shaking. The Reaper paints also separated the most quickly.
Lastly, they also started clogging. I bought almost all of the paint for this experiment brand new just for this, so I was unhappy to find that a few days into the experiment the droppers were already clogging. Overall, I was very disappointed because easily dropping some paint onto my palette or wet palette was a huge plus.
Consistency was actually a little hard to gauge at first, because of the separation factor. P3 has had virtually none, which I think is pretty impressive. Go P3. A quick shake'll do for GW. But Vallejo and Reaper were a pain in the butt, and ended up being somewhat wasteful. I can't imagine how much paint I wasted just on these four models. But it wouldn't have been as much of a problem if not for the dropper bottles, as addressed above. Unmixed elements getting stuck in the nozzles was really a huge downer for me.