Basically the i2cslave/readi2c/writei2c is for the X parts (18X and above) whereas the hi2csetup/hi2cin/hi2cout are for the X1 parts (28X1 and above).
If you have the X1 parts use the 'h' commands. The 'h' commands are more efficient as the 'h' is for hardware as the X1 parts do more of the work in dedicated hardware (when using the 'h' commands) than the non 'h' commands.
Have only used the 'h' commands myself but the syntax and command structures would appear to involve a similar amount of programming work so there is no reason not to use the 'h' commands unless you are using an 'X' part.