I use sleeves and toploaders.
I find cardsavers have a lot of problems.
Removing the card is risky, I've creased a couple cheap ones. Expensive ones I cut out of the cardsaver
They don't fit any of the boxes I use. I don't know where the poster who says they fit all the standard boxes gets his boxes from, but in shoebox boxes the cardsavers are too tall, and they don't fit the 4 row 3200 count at all. (And toploaders don't fit the new ones
)
They're flexible, which is a problem in shipping. I'd rather have a stiff holder with a sleeve and tape than what's basically a fancy penny sleeve.
I don't find either to be heavy. I think a boxful of cards in toploaders would probably be lighter than a box full of cards in cardsavers. Mostly because there's more empty space.
I've sent cards to SGC in toploaders with no problem. It sure beats scavenging the house for leftover cardsavers that Ebay buys arrived in.
Steve B