The B18a is from the 90-91 integra. It has 130hp.

in 1992 the 2nd revision of the b18a was released with 140hp, becuase of bigger cams (more lift/duration), better exhaust and intake manifolds. In 1994 the b18b came with better head design (coolant passages changed) also different intake manifold again - 142hp.

The major differences between them (physical structure):
1. coolant sensor 4 the fan control is on the rear of the block on B18a and on the thermo housing on the B18b
2. driver side motor mount that bolts to the block is different
3. crank pulley on a B18A has a v-belt for the power steering and on the B18B it has a Tri-rib belt for the power steering.

The blocks are almost exactly the same, I would get either - it just depends on which you can find and how much you can get it for.