I second Peters point re all '51 alike, however, the assembler DOES need to know the derivative to limit SFR bit and byte names to those applicable to the actual version. The more advanced '51 user will know that hex code assembled with "XY" as the derivative will run on the "YZ" derivative. Another issue re derivatives is the size of the internal RAM. Now let me say IF a '51 is capable of executing the code the hex file will work. Another point is that the PIC instructions are not as "natural" as the '51 instructions and thus more difficult to apply for a novice.
Whatever you choose, have fun
Erik