I don't know all the answers on why some girls are street girls, and other bar girls. But it seems to me that it has more to do where the girls are from, and how they are recruited then anything else? Let take a bar like Tropical, more than 75% of the girls are from Sinaloa while most of the SGs are from Vera Cruz, Chiapas, Oaxaca, and parts just south of Mexico city.
Now the current thought on the subject on guys who believe they are in the 'know' is that "ALL" SG's or at least 90% of them have padrotes, pimp like dudes, that bring the young girls to Tijuana to sell their body. They put invisible chains on the girls using the threat of telling her parents what she is doing, harming her family or her baby if she doesn't comply, pretty soon the sex just becomes routine and they just do it like robots. The guys in the 'know' will tell you that's why many SGs are so bad when you take them upstairs, and that's the reason you can't take an SG for an 'TLN' is because her padrote will object to it.
I don't necessarily go along with the current wisdom on the subject, as it's gospel on other sites this is what happens. But it is true many young SGs do have padrotes. I just don't put the numbers at 90% and above, and it should be noted the current wisdom differs from what US government agencies tell you, and that's the final destination for these sex slaves isn't Tijuana, but rather the US.( we know that some of these SGs have been working for years in TJ, and they seem to call their own shots, not working when it's cold and going on vacation)
Now the people in the 'know' will also tell you that none of the bar girls have padrotes, because working in bars and having padrotes is incompatible in being controlled by a pimp, as there are too many outside influences with waiters, other girls, and customers turning the girl against the padrote. I don't know if these BGs have padrotes, but many do sure have boyfriends, and a good many BGs support these boyfriends.....call them what you like but they live off the girl.
Now for the question on who is better looking a BG or SG? I would have to say that probably a bar girl overall, but generally an SG is younger with the average age around 19 for a SG, and a BG about 22, a little older when you get them to tell you the truth. While I think on average BGs look better, there are many SGs that are better looking and younger than they most of the hotties in AB, but really it comes down to personal likes, dislikes and the money your willing to pay.