Mediavine names 8,544 sites in its seller file against Venatus’s 618, and 4 publishers run both on the same pages. Reach is only half the question though — the authority spread below shows what kind of sites each one actually carries, which is what determines whether you would be accepted and what the inventory is worth.
Mediavine is the more selective of the two, with a median authority of 29 against 17. The overlap figure is the practical one for a publisher: 4 sites already treat these two as compatible rather than mutually exclusive.
If you want the widest reach, Mediavine is on 7,926 more sites. If you are a smaller publisher, look at the authority spread rather than the totals: the network with more sites in the lower bands is the one more likely to accept a site that is still growing.
You do not have to choose. 4 publishers in this database run both, and the “Also uses” column on either network page shows exactly which ones — a faster answer than asking either salesperson.