Rocco came later thru Clemenza and was "only" a capo. Consistent with the Corleone's propensity for a limited number of capos, Rocco receives the attention in that capacity.
We see Rocco, in both the novel and the film, long before Neri was recruited. In fact, we see Rocco chowing down with Sonny, Tess, Clem, Michael and Tom while they're chowing down and planning the Sol/Mac hit--pretty heady stuff for a guy who'd made his bones only a couple of days (or even hours) earlier.
To my mind, Rocco was security--we see him scanning the crowd at Anthony's party while Neri is nowhere to be seen. Michael dispatches him to get food for Ola's men; Neri introduces himself to Ola and participates in the discussion. In a deleted scene, Michael dispatches Neri to force out Klingman, then he orders around the showgirls. And in the penultimate boathouse scene, Neri's sprawled out, almost leading the discussion on Roth, snickering at Tom's humiliation, while Rocco remains standing.
Bottom line: Rocco was Clem's man, Neri was Michael's man--a parallel to Tom being Vito's choice for consigliere, not Michael's.
