Amazon.com New Releases: The best-selling new & future releases on Amazon
Mythic Odysseys of Theros is the #1 D&D new release. It's already #18 on best sellers list, just behind Ravnica, so I think Theros has a really good chance of overtaking Ravnica.
Ravnica appealed more to hardcore MtG fans, with some D&D fans being attracted to the setting because it was new or for parts to steal stuff from for home campaigns.
Theros on the other hand I think appeals to a much broader audience then Ravnica.
It's not as popular with MtG fans as Ravnica, which is the most popular MtG plane, although still one of the most popular planes.
But it's much easier to grasp and picture by nonmtg D&D fans, many of whom are fans of Greco-Roman mythology, then the stranger world of Ravnica. Plus Theros has a lot more poachable mechanics then Ravnica.
So since it's just under Ravnica right now, which had two years almost to sell, while Theros is still in pre-orders, it seems very, very likely that Theros will pass ahead of Ravnica some time after release. Although traditional D&D setting Eberron and Traditional like setting Wildemount, and even the SCAG are all higher then the MtG books.