Quick and Dirty
No matter what brand of bearing (low or high cost, etc.) you are purchasing, the vast percentage is in your favor to get the same seal or shield material from any company. The reason is that the material is fairly cheap. Some variation of NBR (Nitrile) rubber is going to be the main part of the seal. The metal insert is going to be some variation of cheap sheet steel.
First and foremost, standard seals and shields are just fine for the bicycle industry. The temperatures that bicycles see are well within standard parameters [~-40°F to ~210°F]. Higher end materials obviously do exist, but ask yourself what you are getting out of it before spending the money. Viton, for example, is a great seal material. A significant reason to switch to Viton is for a higher temperature rating, but this is in the hundreds of degrees of Fahrenheit [~-15°F to ~440°F]. Another reason to use Viton or another seal material could be durability. Some may think this makes sense for the bicycle industry; we say, ‘go for it’ if it makes you feel better. But be sure to ask yourself ‘how exposed are the bearings?’ ‘Can you see them from the outside of your bike before disassembly?’ If the answer is no, then what extra durability is needed to handle some water or other dust size contaminants? The effectiveness of the bicycle part design itself will do much more to protect and increase the life of the bearing than the seal material will.
Furthermore! A major aspect of seal effectiveness is the contact level. Verinent, for example, offers two levels of contact: Light (RS) and Heavy (RH). The heavy contact seal is going to do a better job against splashing liquids and fine dust ultimately resulting in a prolonged life of the bearing. Although, the tradeoff is rolling resistance. Expect rolling resistance from a heavy contact sealed bearing (at least for the first 10-15 hours). Any and all seals, rubber/Viton/etc., will ‘break in’ to a certain extent and reduce some of the rolling resistance over time. Shields do not have this issue because shields are always non-contact.
Seals vs. Shields
Verinent defines a shield by metal and a seal by rubber vs. contact and non-contact. The reasoning behind this is simple – rubber seals can be non-contact (RZ) as well. Simply defining shields by ‘metallic’ and most other materials by seals keeps it simple and reduces confusion. If you demand it, a step deeper could be rigid vs. malleable. This is because a PEEK, or PTFE material would also be considered a shield, not a seal. These materials are very hard and function the same as a metallic shield rather than a rubber seal.
For the bicycle industry, we would generally advise to use the RS light contact seal. The reasoning is that you get some contact seal protection, without much resistance (won’t be noticed). If you prefer cyclocross or mountain biking through rivers in a torrential downpour, then go with the RH seal. Just take the bike for a few rides before taking part in any competitions. This will ensure that the initial break in period is past before needing to count the seconds of your win.