I think Marcus is our only safety with a "nose for the ball" as far as interceptions. Bell has it in the run game. Marcus didn't make as many plays as he did as a rookie, but "sophomore slumps" are common, especially for players who have stellar rookie seasons. Marcus still flashed his ability in 2018, and I expect him to have his best season yet in 2019. If that happens, he will be one of the best safeties in the league, because his 2017 season was that good.
Marshon is in the same boat. Both he and Marcus regressed a little with their greatest assets. Playmaking (interceptions) for Marcus. Blanket coverage for Marshon. I expect both to be near the top of the league in year 3.