If you currently have a functioning PS2 BIOS from a 50001 or 70001 model, you likely won't notice a massive "performance" boost by switching to the 90001. The PS2's internal architecture remained consistent enough that games don't run at higher frame rates just because the BIOS is newer.
The short answer is that while the SCPH-90001 BIOS is the most "refined" and compatible with the final hardware revision, "better" is subjective and depends entirely on your specific use case. Understanding the SCPH-90001 BIOS Architecture
Better handling of the internal clock and memory card management.