So I am considering another transmitter and receiver, to supplement my B block units.
I have 2 transmitters (one stick mic plug in) and one receiver.

I could buy another trans/receiver setup, or just one receiver.

Should I get another B block, or would an A block provide more flexibility?

The USA use blocks A, B and G (G is between A and B).

If you want to use the new receiver with your existing transmitters you will need to get it in the same frequency block.

I'd get it in the same block as your existing. That way you can transmit one mic to two cameras if you ever need to. It makes it really easy to sync two cameras in post when the waveforms jive.