A lot of your chances into business school depend on which business school you want to attend. You haven't mentioned that in your post, and as you already know, there's a huge amount of variance from school to school. A higher GMAT will definitely help your chances. My only warning is to not take the GMAT too many times. If you can show a progression of scores, and your next one is higher than the first, then it's good. But if you slide backward and your scores start to fall, then it makes it seem like your one good score was just luck.
The MIS vs. MBA is a big difference. An MBA will train you to be a manager and administrator. You will manage other people and administer over a business unit (eventually). An MIS degree will position you to do this specifically in an IT capacity, or it might even provide skills to be a high level individual contributor. These are very different outcomes.
While it sounds like a lot of work, if you're REALLY conflicted, you could even apply to both types of programs to give yourself some options.
Hope this is helpful,
Former Kellogg adcom member, offering a new approach to MBA Admissions Consulting.