Hi emiazz, I can provide you with some input as a recent graduate from SIM. Hopefully it will help you to make up your mind, cause I know how tough this choice may be.
First of all, why do you have to choose between SIM and CEMS while you can do both at the same time? You'll need to apply in the first semester, go through a few test/interview rounds and, shall you be successful, you'll get a double degree in 2 years. A couple of my group mates followed this way and are very happy with their choice, cause CEMS definitely offers great opportunities for networking as well as numerous corporate events. But I should say that even without CEMS, SIM program is already very international in terms of its students' composition and general orientation. For example, we had a relatively small group in my generation with more than 20 nationalities present there, which is quite an intercultural mix in my opinion. Also an international internship would be a requirement of the program which also helps you to enrich your international experience.
Regarding life in St. Gallen, I can't claim that it's better than in big cities like Stockholm or Rotterdam. But it's certainly not as expensive as you probably imagine - you can live quite a comfortable life here if you are not throwing money around on some stupid things. And it's definitely not boring, cause there are always events organized by various students' associations which are numerous in HSG. Also SIM curriculum will not let you feel bored, that I can promise
Your profile seems to be quite good for SIM given their entry requirements, so you should definitely give it a serious consideration. However, it would be good to know what other expectations you have from the Master degree (other than international orientation and enjoyable lifestyle), cause this may also be very relevant for your choice in the end.