Is there a substantial difference
No but they are different.
The PCA9555 is a more modern chip with more registers and improvements. For example it has a polarity reverse register where you can decide on a bit by bit basis if writing a logic one to the bit produces a high or a low. Quite handy for when you are mixing sourcing and sinking devices, so you can get a consistent a 1 turns it on. You also have 8 more bits, it will run at 400KHz if you hack the library, but it is probably more expensive.
The PCF8574, has been around a long time, it was one of the first I2C chips. It has less in it and therefore it is simpler simply because there are fewer options. It will only run at 100KHz.