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wesleyAdmittedly, I was very much a slabhead the last few years. Recently Mark Macrae and Julie introduced me to mylar holders. They're similar to Cardsavers in that they are semirigid, but they were originally intended for preservation of stamps and paper documents. The mylars are clearer and apparently safer for paper than traditional Cardsavers. So from now on, my cards will be in the mylars.
As far as grading companies, I would trust SGC, PSA, and GAI equally and I would purchase cards encased in any of those holders. So to answer your original question, no it does not matter so long as the cards are graded by one of the three reputable companies. If grading is still in vogue when I need to sell the cards, I will go ahead and slab them at that time.