Is analog output more accurate than RS232 output?
In the general case, there is no answer to your question. Most likely, the RS232 output is digitizing the same analog output, so the question is really this: is the built-in D/A conversion less noisy than whatever external measurement equipment would be used for an analog signal? The answer will depend on both the sensor and the specific measurement equipment being considered.

In this particular environment, I bet the reason the analog output was chosen over RS232 was that it was easier or cheaper, and good enough. Like good programmers, good engineers are lazy.