It is not an adversary relationship. The major difference in implementation is that a 300B will give you about three times the amount of max (practical, what-have-you) power out. If I would characterize the 'sounds' of the devices according to my experience:
300B; romantic, lush. If a fault, histrionic.
2A3; clear, clean, tight. If a fault, dry.
But; it all depends. On the amp, the pre, the source, the speaks.
You said you have FE168Sigmas. Which ones; the new ones with the Escherian/Fullerian cone/surround/cap, or the older model with the exponential cone and whiz?
Poinz