I agree with other posters that your decision likely pivots around how much work experience you have. Beyond that, I think you have to examine what's necessary to succeed in your desired career.
I assume by business development you mean some type of sales function. I'm not sure how this works in pharma, but in most other fields success in sales is typically really about being able to sell more and more business. The best salespeople I've worked with are frequently much less educated than their peers but they're driven, persistent, know their products and how to relate to their customers.
For that reason, I have to wonder if you'd be better served making the transition within your own organization and getting some early depth in this space before making a decision about how/where to get your MBA.
I'll also throw out that there that sales positions themselves aren't hard to get--since most business development positions have a significant emphasis on variable compensation, there's a perceived lower cost to a sales hire. My experience has been that the organizations I've worked for are much more willing to take a risk on a BD career switch than in other functions. Meaning, I wouldn't think an MBA is the key to switching gears to sales.
I'm not saying an MBA won't benefit you, just playing devil's advocate about whether it will help your mission to move into a new role.
Good luck either way. If you go full-time, you'll probably have a lot of other opportunities to make sure whatever career you target fully leverages your degree.