At the risk of generalizing, the 300B can have better frequency range and a more "transparent" midrange. The 845 will outclass the 300B in bass fullness, although in some 845 amp designs, the bass is bloomy and bloated. The 845 was never designed as a wide audioband tube.
In my personal opinion, if you don't need the power, I'd go with the 300B (or 2A3 or 45 even). If you need the power, I would also consider the 211 triode versus the 845.
In my personal opinion, if you don't need the power, I'd go with the 300B (or 2A3 or 45 even). If you need the power, I would also consider the 211 triode versus the 845.