I'm a software engineer and last year I finished my company-sponsored MBA degree. My opinion is that between MSIS and MBA, MBA would be a much better option. IMO MIS (and MSIS) is more for people who want to learn computer applications and business, if you are already a programmer, you know WAY more about computers than what the school is most likely to teach you, and you probably have the skills to quickly pick up new software packages even if you are introduced to something you've never seen before. MBA is all business and as a computer engineer, you might as well go for business, because you either know, or can easily learn, the IT side of things.
As far as workload, MBA doesn't even come close to the amount of work I had to chug through when getting my BS in computer science, but it does shed light on things that a typical engineer wouldn't consider. It's true that you don't need the degree to become a manager and plenty of people without MBA have been promoted very high up. At the same time, as Peopleware (I believe that was the book, but not 100% sure) pointed out, engineers make some of the worst managers because as engineers we are used to being able to control things we work on. On the other hand being a manager is about making decisions that don't have solid outcome, and often there's no formula which dictates that if you have a specific set of inputs you will succeed. And you have to deal with people which is another big, unpredictable factor. MBA will force you to think and consider these things so hopefully you won't screw up as much when you do become a manager.
MBA really does give you a different outlook on life. I can now justify 25% of my company's upper management's decisions even though to an engineer they'd look plain silly. We also have people at VP level with no MBA and when they make changes that the first class about Managerial Accounting and Control teaches you not to do, it's very amusing to see the outcome that's completely predicted by what the theory says you'll end up with.
Another huge factor to consider, and this is something I royally screwed up on, is you go to MBA school not just to learn, but to make contacts. If you decide to go back, as painful as it sounds (and actually is), try to work with as many different people as possible. Most classes are group work and you will meet executives, business owners and other engineers such as yourself. Making contacts is tough and school will give you access to 100's of people and will force them to get to know you and vice-verse. Working full time, I didn't have patience to deal with others, so I had few friends that I stuck with for all 20 classes.. thinking back, shouldn't have done that.