Is it safer to have a prepago girl that has been reviewed on this site come to my place or is it better to go out to the spas.
Those two are not necessarily different things. Some of the girls reviewed here are spa girls. Let's just assume you are only going to stick with reviewed girls (spa or independent) because random ads are risking too much both with quality/safety.
In that case, by staying in your hotel (with security checking IDs and not letting the girl leave without checking with you first) I'd say that is a little safer as you don't have to go to a location that you are unfamiliar with. I would give that the edge. Also understand that this will be much more $. Relatively still not expensive (so I still did it a couple times).
However some places let girls leave without checking and the girls know it. Other places don't have security at all (some AirBnBs). This would be less safe as she could get your stuff. Again, with reviewed girls I don't think this is AS likely, but if you have valuable shit lying around then it might be worth it for her to take a chance. This would be less safe IMO.
As for reviews: Just keep in mind that if there is just one (usually positive review) it might be a self-review. Stick to the ones with multiple positive reviews. Especially from people who post negative reviews (when warranted) also.
Also keep in mind that we all have different tastes. One guy here likes them bigger than I do. My 10 might be a 7 to him and visa-versa.
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P.S. Understand that the popular ones are really tough to get a hold of so you should probably start reaching out a least a few days before you get to Medellín for highly reviewed independent girls.
Feel free to post which girls you are thinking about and the guys here can give you follow ups. I recommended a girl for a while but I've heard from multiple people that her service has went down hill. This is something people are also talking about on the Spanish speaking side of this forum.
As I said above, we all have our own tastes but perhaps we can keep you safe.