Is this dead already? I am disappoint.
My take:
Spongebob versus Xenomorph
Dragon Slaying: Advantage Xenomorph, for being able to fight.
Maintaining Audience Interest:
Dogfighting: Both would crash their Jets, i just don't think a damp sponge is going to survive the explosion, the Xenomorph possibly could.
Spongebob actually wins here, against all odds; they're 1/1 going into the third round (spongebob being the better entertainer on account of not immediately trying to eat the audience), but the
filthy Xeno
scum has to forfeit the third round on account of not being able to fly a plane (whereas spongebob is at least theoretically capable, even if he'd crash it and die horribly)
Captain Malcolm Reynolds versus A Dreadnought
Memory Games: I would theoretically give this one to Reynolds, as he's younger than most any Dreadnought, who must all eventually be put out of commission as their brains slowly deteriorate.
Spying: Mal can be subtle, five ton cyborgs might have a harder time.
Building Robots: Dreadnoughts come from a much more technologically advanced society, which instantly gives them a leg up.
Not going to call this one, but a couple things to consider about the Dreadnought (don't know anything about the Captain):
1: The deterioration you mention takes a lot longer than you think it does: The oldest living Dreadnought (who's also the oldest living Astartes in the Imperium), Bjorn the Fell-Handed of the Space Wolves, is older than
ten thousand years, and even though he's been sleeping for longer and longer periods, he can still remember the pre-Horus Heresy days, having fought alongside the Primarch of his chapter, and seen the God-Emperor of Man before he was interred onto the Golden Throne.
...But all that's irrelevant anyway, since this is implied to be a short-term memory contest. Given that Astartes are genetically engineered super-humans (even healthy ones can live for millenia), and assuming the Dread is only a few centuries old, unless Reynolds is known for his exceptional memory, I'd give this part to the Dreadnought.
2: While a mech the size of a monster truck is un-subtle as
fuck, I wouldn't rule the Dread out of spying completely: debatably, the sarcophagus' machine spirit could help the dread interface with other technologies (computers, security systems, etc.) for alternative forms of snooping.