The choice of "C" or assembler is not universal. If you have VERY critical real time issues, they must be in assembler as most/all "C" compilers save EVERYTHING entering an ISR, this is very safe, but consumes processor time. On the other hand if you do not quite know what you are doing, that, of course, is preferable. If execution time is a moderate concern, it MAY be the best approach to write the ISRs in assembler and the rest in "C". If the code in assembler cannot be squeezed below 4.056k, MAYBE you might as well have used "C" since you need 8k anyhow.
The main advantage of "C", portability, is not of much interest in the embedded world since a move from a '51 to a '52 can easily be done with assembler code and a move from a '51 to e. g. a PIC is not likely because of hardware concerns.