assembler or C?
It doesn't matter. What matters is that if you approach the programming with a C attitude (the compiler will take care of it for me) you will fail.
If you choose C and decide to forget about DATA, IDATA and XDATA you will fail for sure.

In other words, regardless of whether you choose assembler or C, you need to understand I/O and memory access/types at the hardware level.

In PC C the mantra is "the OS takes care of I/O", if you were to e.g. implement Linux on a '51 to achieve the same, you would end up with a system so bloated that it would have no chance in real life. Thus you need to understand the very same regardless of whether you choose asembly or C

Erik