Kabul:

Looking at Stella's Leo 2A6 and M26 Pershing sets, they appear to be metal-shielded units. And as the Sherman ones are mostly enclosed, I'd say you're fairly safe with them. As I said, I prefer them as they can be cleaned & relubed easily, where the others are next to impossible without ruining the seals.

It all depends on your preference, I guess. Metals are cheaper, teflons are most expensive.