Disclaimer: I did my undergrad in St. Gallen.
As stated before St. Gallen beats Mannheim with regards to reputation in German-speaking Europe (which is practically one job-market) - if you consider the whole institution and quality of academics. And that's probably what the 50% uptick is for.
The real question here is about the MBA degree - it is an American invention and very well accepted in the English-speaking world. But the situation is really different in Germany and Switzerland, where an MBA just doesn't really fit with the traditional educational environment.
I believe therefore it is impossible to say which one is better in general terms. There is no established and accepted "reputation" for the MBA programs. It really depends on what your pre-MBA experience looks like and what your goals are. But it seems that this is not entirely clear...
So: Mannheim is cheaper and you'll find it easier there to learn German - I agree. But frankly, learning German is not a one-year undertaking, especially if you do an intense MBA program. Unless you are an absolutely outstanding (and I mean absolutely outstanding!) language learner and you know that you'll pick it up "on the go" I assure you won't be able to speak decent (i.e. useful for business) German after one year. So it don't think this should be a deciding factor.
In you're case I would really go and visit both schools and check which "vibe" you prefer. The Germans in St. Gallen for example, usually like that its close to alps for skiing etc. but on the other hand it's a small town... you like it or you don't.
If you don't want to visit the schools I would go for Mannheim. 50% higher fee is too much risk for a school that you might actually like less.
Hope this helps a bit.