Magnacore: Excellent color across the entire dynamic range and they project really well. Response is very quick, and yet they still hold their composure when you put the hammer down. In my experience those two qualities have been mutually exclusive, but Larsen have pulled it off.
Spirocore: The color is a little more "raunchy." Not unpleasant, but definitely edgier and with better projection. They're just as powerful as the Magnacores, but with two drawbacks: I feel like they lose definition when playing very quiet, and when hitting hard they feel brittle and unstable. Within minutes of playing them, I had already resolved to put Magnacores back on when budget permits.
So are the Magnacores really "Spirocore killers?" To me they totally are. With the exception of a slightly less projection, they are better on my cello in every way. And I feel like my cello is very honest and provides good universal feedback; it's got a big, broad rumbling tone and isn't finicky about what goes on it.
If I can ever afford it, I'd love to try the Jargar Superior G and C, because the A and D that I just put on are phenominal......