Tough one to call simply because they're so different skills wise. Both like to eat punches and are lacking in their defensive abilities, both like to push the pace and go forward looking for the finish.
Although Spencer is definitely the better grappler (she's got skills, physical strength to hold someone down, and is quite sneaky in how she can isolate an arm or head/neck), she's facing someone who is just a physically strong or even stronger. She'll have to be very smart in how she sets her takedowns and clinch, instead of just rushing forward or throwing kicks hoping they land.
We all know Anderson's ground game is no good, her takedown defense isn't exactly stellar either. However, she hits REALLY HARD and isn't afraid of banging it out with her opponents. She could catch Spencer clean with a few shots or kicks, and knock her out, her power is no joke. Megan needs to be smart and keep her distance/counter, as Spencer is more likely to come forward in a desperate attempt at scoring a takedown.
If we judge Spencer by how she looked in her last outing, Megan will knock her out cold if she can't score a submission win in the first round. Sorensen was lighting her up in the stand up, Spencer had nothing for someone her own size, now a larger and physically stronger opponent will really hurt her if she goes out there fighting with desperation. Or I could be wrong and she submits Megan fairly easily, you never know with this sport.
Let's go ladies!