Keller has been worth 7.5 fWAR through 523 IP, which is a total for maybe 3 seasons, making him worth roughly 2.5 fWAR per year. Bello has been worth 2.9 fWAR for his first 214 IP, probably 1 pace for 1.5 seasons, pacing him at about 1.5 fWAR per season to date. Bello has not been as good as Keller to date, but I think he can outperform him over the next few seasons, assuming no injuries. But the risk of injuries are also why the Sox can get away with a lower salary with Bello; he is in a much riskier position.