I don't think you can go wrong with this choice here, both series are absolutely great.
I would say that Mistborn probably has more mystery, intrigue in twists (not that Stormlight doesn't, but the twists/mystery in Mistborn tend to be more of a focus and more impactful). I would also say Mistborn will provide a bit of a clearer understanding of the Cosmere than Stormlight, so it is generally what I recommend first.
On the other hand, Stormlight definitely has greater focus on character development, and their personal struggles. It's also just really an incredible series (so even if you don't read it first, you should still eventually read it). Also, you mentioned that you usually read one series and take your time doing that, I will warn you that the Stormlight Archive series is only on book 4/10, and book 10 probably won't be released for another 20 years or so.
Finally, for strong female characters. This is my opinion, and others may disagree, but if you are looking in-particular for strong female characters, I would look somewhere else. Brandon Sanderson's books are great, and I absolutely think you should read them, but I have found that his female characters feel consistently less or underdeveloped compared to the male characters. That's not to say there aren't strong/powerful/smart female characters, most of his female characters are, and most of them are independent of men, but the character development is lower quality. I'm happy to provide recommendations of other books with strong female characters if you want, or for after you're done with Mistborn/Stormlight.
