If one were to choose between these two as a form of sexual release only for the sake of pleasure, which one would be safer on a spiritual aspect?
Keep in mind, that prostitution is illegal in many countries and obviously ToZ doesn't advocate things against the law. But whatever happens between two consenting adults is their own business.
Natural will always be the better option and its obvious. With porn, the mind becomes accustomed to something unnatural, the brain starts rewiring towards quick and easy dopamine pathways, and If it becomes a habit, life will become sluggish.
There are a bit of things to become aware of though. Mainly the fact that not only is it illegal, but this can put you in the crosshairs of ongoing investigations and even get you in trouble with other gangs, armed pimps etc.. When these women get busted by LE, their goto motto is often that they were coerced or forced, regardless if true or not, to in order to avoid harsh penalties, especially by the IRS. Then a deal is made and their pimps and/or contact clients are handed over.
As said above, its preferable to meet clean and reputable women for consenting moments and not some craigslist street girl. Quality tends to be on the pricier side though, and one could end up burning through their wallet if they're not careful. Stds will always be a risk.
I know the ethics of this is heavily debated, and I don't want to make an essay of this, but we have to agree that prostitution flows through the natural side of things, in which the universal law applies that one must give in order to receive. Same is true in the dating game in which expenses and time must be made or order for things to advance to the next stage. This is basically getting to the point, but we must also respect the laws. Quality women will encourage getting to know you, carefully choose who they meet and will always make safety a priority. Also, you want to leave with a good conscience in in knowing that they will put their money to good use.
A final note, the path of Zevism and the original intention is that we evolve to our higher selves and become beyond our primitive tendencies. What makes us "human", that we are in control of our minds and not the other way around.