As stocks seem to be dwindling, I decided to pick up the official Dagu Rover 5 controller board while I still could. This uses SP8M3 FETs to form the H-bridge, which should only drop about 0.3V, as compared to probably about 2V from the crude L298 drivers I used last time. The board also mixes the two quadrature encoder outputs into a single waveform using a XOR gate, which reduces the number of pins I need. There's also a nifty current sensor, just so I can keep an eye on the current consumption and shutdown in case something bad happens.
The only downside is that it's all 5V I/O.