I highly suggest going the technical route. Ask yourself this: would you prefer to be a software developer in a start-up (including your own), or a manager in a more established firm? If the only sort of management you see yourself doing is in <i>your own company</i>, I don't see an MBA as being very useful to you. You need to be some self-sufficient, meaning you should know how to code and not <i>just</i> how to manage. It's easier to bring in outside managerial talent to a purely technical firm rather than vice versa (I personally would <i>never</i> be the first programmer in someone else's company, many other hackers are similar).<p>You already have a strong background in a quantitative discipline (CEng). It means you'll have an easy time picking up either. Don't forget the advantage that this gives you. That means you'll have an easier time with statistics, mathematics and numerical analysis: this opens up a whole new world for you that's closed to many programmers who are only good at programming.<p>That being said, there's a lot you still have to learn. Start with learning C (you might not develop your application it, but a) you need to understand how physical memory works in order to be a programmer b) C is the lingua franca of programming languages which means you'll have no problems learning whatever <i>other</i> language is better for the task at hand), then learn algorithms and data structures and at least three of (compilers, operating systems, databases, computer networking). Then specialize: machine learning/NLP/IR (given your "real engineering" background, you might have an easier time here than most programmers), further into systems, further into databases, etc...<p>That's a multi-year project and you have to be ready for it.
Don't under-estimate the value of experience (by experience I mean working in firms whose product is software i.e., <i>not</i> consulting or internal IT development) in doing a start-up: it's possible to start without "having done it before", but several years of work experience (alongside people much smarter and more experienced than you) is extremely useful. Experienced developers have a huge role to play in a start-up: partly by making it possible for apt junior developers to be recruited (they want to work alongside "hard core" hackers) and mentored.<p>If you join a start-up, it's also a chance to be exposed to the "business side" as well -- although certain types of established firms (in my experience, those who build software for other developers) will give you exposure to customers and sales/support cycle as a developer too.<p>If you prefer a more structured environment in which to learn (as do I -- there is absolutely nothing wrong with that), an MSCS will serve you well. It's also a chance to meet hackers, show your "worth" to them (they'll have collaborated on lab projects with you) and make connections without looking like a "shmoozer". It will also give you credentials for employment (getting initial experience), although I'd have zero hesitation in hiring a self-taught hacker with a degree in Civil Eng (software development is really an engineering discipline rather than a science).