Are NGC slabs better for 1936-42 Proofs?

In my search to acquire a full 1937 Proof Set I am have noticed that many coins in PCGS holders are stained and toned, while thier NGC counterparts are nice and fresh. Would this be an indication that NGC holders are better to preserve the coin and protect them against toning and other enviormental factors?
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Comments
ngc hands out cam too easy
<< <i>i've seen the cream to be in pcgs holders
ngc hands out cam too easy >>
Buying top quality Seated Dimes in Gem BU and Proof.
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Michael Kittle Rare Coins --- 1908-S Indian Head Cent Grading Set --- No. 1 1909 Mint Set --- Kittlecoins on Facebook --- Long Beach Table 448
We are not asking about the grading merits of the grading services. Halfhunter is just asking about which slabs preserve coins better.