I suggest starting with the 300B. The 2A3 arrangement is direct coupled, which is a more tricky approach; the operating points (voltages and currents) of each stage are critically dependent on the other stage. There are just more ways things can go wrong.
As a side note, you can operate the 300B in direct coupled mode, by changing the audio circuit but leaving the filament power alone. This will give you a good part of the nuance available with the 2A3 in this amp, and the 300B will have a longer life when loafing at 15 watts. Many prefer the JJ 2A3-40, which is basically a 300B with a 2.5v filament.
All that said, of course the current Paramount has the new 5670 driver with the new "soft-start" board, which allows adjustment of the critical driver plate voltage. This solves one of the tricky tricky issues the direct coupled circuit.