Although I never used either when I was working in uniform, I talked to a couple co-workers who do about this. It basically comes down to personal opinion and the types of situations you feel are most likely going to occur when you need to access a backup firearm.
One reason that a couple of my ankle carrying coworkers site is that if they are taken hostage and their hands are bound behind them, they could still access the weapon in the ankle holster. They are also concerned that if they have a physical confrontation with a bad guy where they is some type of wrestling, the bad guy may feel the vest holster and become aware of the hidden weapon.
Bottom line- try each yourself and see what you think. The vest holsters seem more secure than the ankle holster, but neither will do you any good if you cant get to them easily.
Also, no matter which one you get, consider having a tailor make some alterations to your uniform to make accessing the weapon easier. One of my coworkers with an ankle holster had a tailor seperate the seem of his pants by the ankle and make a velcro closure there. The pants leg appears normal, but if he needs to get to the backup weapon fast, he just tugs the pants leg open for quicker access to the holster. I would think same type of thing could be done to a uniform shirt to make a quicker access to a vest holster.
Joe Mech