MIS and MBA are two very different degrees, so which one is best for you depends on what you want to do. If you want to stay on the technical side of IT and become more of an expert in the field, I think MIS is best for you. If you want to get more into the business/management side of things (or want to change careers entirely) the MBA is best for you.
With regard to an MBA from a non top 20 school, they have plenty of value. Of course you should try and get into the best program you can, but at the end of the day an MBA from a school ranked 20+ will still present you with plenty of opportunities. Will they be the same opportunities you would have from a top 20 or top 10 school? Probably not - especially in fields like consulting or banking. But even so you'll get a very good education, and still have very good career options.
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