If I were you, I think I would choose to go to the University of Pennsylvania. I have no doubt that both programs are great but there are a few factors that UPenn has that may be more beneficial to you.
One of the biggest reasons why I think UPenn will be a better fit is because of their environment. Don't get me wrong, BYU has a fantastic atmosphere. For the most part, I think we’re a pretty positive group of people. However, Provo isn’t your average college town. You’ll have friends from school and the people you live around, but a lot of our social scene revolves around the Church and ward functions. If you're not LDS, it can be hard to form those social bonds. If you go to UPenn, I'm sure you'll grow close to the people in your program, but you will be able to branch out and meet other people. There are just more opportunities that will naturally present themselves there.
Additionally, from what I know and what you described, if BYU's TESOL Master's program is prepping you for a role in administration, be prepared to continue in graduate school. Most positions of higher authority in the school system will require a specialist or doctoral degree. Also, at BYU, you're granted a Master of Art while UPenn will hand you a Master of Science. To be honest, I don't know if employers consider this important, but it may be something to look further into.
Like you said, prestige is a huge part of Chinese culture. Even after experiencing it myself, it's hard to wrap my head around how much it is valued. While I consider both schools excellent choices, UPenn is a Top 10 university while BYU is in the Top 100. Both are great rankings, but if you're looking for the well-known reputation, then Penn is definitely for you.
Lastly, if you're leaning towards BYU, be aware that the university will always be associated with Mormonism. Your friend is right: there are some people who get hung up on the fact that it is a Mormon school. However, for the most part, I feel like the majority of people in the States are rather accepting. While I was in China last summer, I had some people ask what school I went to. I would tell them a little bit about BYU and my description would sometimes include that the university was supported by my church. Did I tell them what denomination? No. Were they interested? Not particularly. They were content with knowing that it is one of the best universities in the nation with a large student body.
The only true deterrent I can think of for the University of Pennsylvania would be that big price tag. It's not a secret that teachers get paid poorly. In America, we get pay raises based on how long we've been teaching and how many education degrees we have. I'm assuming it is similar in China. I would really have to consider if the social atmosphere and the reputation of the school is worth the price difference.
You are the only person who can decide which school works the best. Really consider what you want, what you're willing to sacrifice, and which school will best fit your needs.
Best of luck!