[MattyC]

Here's the bottom line:

Chris above pretty much nailed it, for my money.

If the cards are beautiful with eye appeal, most collectors will see right through any holder like the clear plastic it is. Holders are surely helpful for preservation and authentication, and the Registry game is a very specific type of collecting all its own, but end of the day savvy collectors want a great card-- and they can judge one in any holder. Or none at all.

More specifically to your questions:

I would only get an existing raw card graded if I was going to sell it, and if the realistically expected grade would substantially increase the sale value.

For 1950s, assuming you are selling and thus grading, a gorgeous star will perform well in either holder. Commons I must say the PSA Registry collectors will pay more for a high grade common.