You can't send thick cards (GU's, Blanks that are signed, etc) in a card saver and a thin bubble mailer and expect them to arrive undamaged...thick cards get damaged by the sorting machines at the post office and crease easily!!! DUH!!!
top loaders are at least a little rigid...card savers are thin...like mylar...and may be just fine for thin cards...but the thicker cards like the GU's need some sort of support...it's easy enough to just tape two pieces of cardboard around them a little bigger than the size of the card.
I don't use card-savers period. It's just my opinion, but sending something in one of those things is like sending a card in a penny sleeve and shipping it out. It might work fine, but I don't take that chance.
For plain single cards, I always use a penny sleeve and card saver/semi-rigid holder unless the other person specifically requests something better. Lets me use a PWE without they worry about machine damage that a top loader would invite. Just flexible enough to make it thru the machines (little different than a birthday card), without having to worry about edge or corner damage. Some 200+ shipments later, I've never had an issue. But I wouldn't think of using that arrangement for any thick cards.