Open bearings are typically only used in clean oil bath applications where secondary sealing methods protect the bearing from external contaminants.
Metal shielded bearings have a small air gap between the inner race and the metal shield allowing oil to pass through to the rolling elements while protecting it from larger particles. However, this small gap can be a liability as it allows dust, dirt and moisture to pass through and contaminate the bearing; therefor they should not be selected for use in contaminated environments.
If a completely sealed environment is required, the rubber sealed type is the best suited choice as the seals lips physically contact the bearings inner race.
Open and metal shielded bearings are capable of higher RPM speeds than rubber sealed types as there is no physical contact between the bearings inner race and the metal shield.
Timken Corrosion Resistant Deep Groove Ball Bearing Catalog