All in all, you're taking a really good approach to the matter.
Probably the best advice I ever heard about job/career choices is "What would you do even if you never got paid for it?" This isn't to downplay the importance of career advancement and salary. Rather it is to emphasize that no one can pay you enough to do a job you really hate. Maybe you would equally enjoy both options, which is fine, and your decision might come down to more esoteric considerations.
All that said, the BI field is larger than just Microsoft. There are principles of BI that have nothing to do with platform or vendor. Certainly MS is a major player in the market and you really can't go wrong learning their stack (after all *somebody* is probably going to be using it), but if you focus on learning the underlying theory and principles, then switching to a different platform/stack won't be all that difficult.
These days, business as a whole seems to be more data-driven as demonstrated by the emergence of such fields as data science, predictive analytics and so forth. Business has been accumulating all this data into various databases for years now and they want to do something with it. So from over here in the cheap seats of the peanut gallery, I'd go for BI in general all else being equal.
On the flip side, app development certainly has it's attractions. Gathering, manipulating and storing data isn't necessarily the same skill set as creating useful interfaces between users and all that data. As you point out it is larger and more saturated, but that doesn't mean you can't distinguish yourself. I see posts in various forums all the time about young developers who seem to come unhinged when reality doesn't follow the textbook. You've obviously got some practical experience, which is a big plus. Personally, I'd rather hire someone who may not have all the academic credentials but possesses some real-world business experience over the latest graduates in computer science.
At any rate, here's to your efforts. I honestly think you'll make it whichever way to decide to go.
Just my $0.02 from over here in the cheap seats of the peanut gallery - please adjust for inflation and/or your local currency.