Fight on.....even when he eventually beats you (afterall he IS Magnus) think of how much one would learn by continuing to play and observe his strategies.
if you were a piece down against Magnus?

No different than if two pieces up against him. Be sure to know your blood type because shortly you would be needing it from the savage beating about to befall you.
* - one of the more sorry ass forum topics - a great boredom seems to have settled over our unimaginative flock of young patzers
If i was a piece down against Carlsen i would resign, to play on is a terrible way to disrespect a player of his ability.

I would go into deep think for a while and then sac a piece to try to put him in check, just so I can say to the world champion with determination "CHECK!". And when he just got rid of my pieces, I would raise an eyebrow and go "hmmm, interesting", as if I miscalculated my checkmate attempt
If i was a piece down against Carlsen i would resign, to play on is a terrible way to disrespect a player of his ability.
Meh, it comes with the territory of playing down so low. I wouldn't treat him any differently. I would already be kind of embarrassed just to play a game... but if we're already playing then I'm not going to resign any earlier than I normally would... a whole piece isn't quite enough for me to resign in a serious game (I usually play a few more moves).
would you resign or play on?