Namor the Submariner solo film would not compare to a Silver Surfer solo film, IMO.
The surfer has already been exposed to non-Marve viewers, ( FF: Rise of the SIlver Surfer) ,..has had his own casrtoon series, and you can expand so many more other Marvel characters into the mix more than you could with the under-sea dwelling Namor.
Plus i still have concern that Namor is too close in competitive comparison to DC's Aquaman who makes his appearance in the upcoming Justice League movie.
Namor would work best with another superhero group (ala FF4) but not solo.
A far cry from the Surfer who could work in Galactus, Thanos, Adam Warlock, Captain Marvel, the Kree, ....of course the FF4 continuation, the Elders of the Universe, any of the other heralds of Galactus, Ego the Living Planet, etc. etc