Hi,
I spent most of my career in finance (investment banking, sales and trading, and investment management marketing) before going to the dark side of admissions consulting. When it comes to the MBA vs. a Masters in Finance, I think in all cases, it really depends on what you want to do.
Very very very briefly, the master in finance is far more quantitative and has a lot of focus on mathematical modeling. When I worked at a large investment management firm that was focused on quantitative methods, I saw a lot of masters in finance along with PhD's in stuff like physics. My MBA didn't hold as much currency there as it did when I worked in investment banking or even in sales and trading.
Then of course, there's the question about CFA rather than the MBA, which I believe is discussed somewhere else on this board.
The MBA teaches lots of finance courses, and you can concentrate in that subject at some schools. The experience is different because the MBA is much more holistic. I could go on and on, so if you have any specific queries, feel free to PM me.
As for the specific question of getting an MS in finance first and then an MBA, I don't think that is that useful or common. I suggest picking one or the other.
Best of luck!
Betsy