Congrats on getting into both programs!
I have a feeling Booth is a lot better ranked than St. Gallen. It is basically a Top 5 in the US/World. It is a great long term investment.
I don't know the reputation of St. Gallen but you will get a big advantage being on the ground and making connections with your classmates and networking in Zurich, where you want to be. At least one of my friends have networked with EMBA classmates to go work for several of them or with several of them (after those extended offers to them).... he changed jobs in a year but still, EMBA connection provided him with an option to get a new job.
If your goal is not to get a new job per se but a promotion in your current company, it may be a different story. Or if you think you will move to a diff place, then absolutely Booth.
P.S. I thought IMD was the Top program of choice in Switzerland...
P.P.S. Have you visited the MBA campus? I have heard St. Gallen main full-time MBA campus is fairly small and tucked away, not presenting itself very well.