I would only favour Serena at Madrid but even at that I would have to see how the new clay plays first.
Serena does struggle to move on clay though, her speed has always been a muscly effort as opposed to a more natural finesse about the court, and her footwork is piss-poor most of the time so she isn't comfortable running around. In fact, any player that gets her moving at all or gets a good amount of spin will get a weak/midcourt response from her on the surface. I just don't see how she could cope with an in form Maria on clay, who would be consistently hammering back the balls with a lot of spin and implementing the angles when she got the chance.
Plus her serve on red clay isn't anywhere close to the weapon it is on hard/grass.
Yes Serena is the best on tour, but people think her RG showings are purely mental, which it largely is, but there have been a lot of other red clay tournaments since 2002.
She is the GOAT of this generation but she hasn't bothered improving on clay ever, and her limitations on the surface shows each year.
I really want them to play pre-RG because if Maria can get a win, and then hopefully goes deep at RG to meet Serena (who would have to break her QF curse) then she shouldn't let her head prevent her from winning.