High Sierra VS Catalina?

Depends, does 10.13 not do what you need?

Do you have the funds & do the developers have the 64 bit Apps o replace any 32 bit Apps you use?

To see which Apps will no longer work in 10.15 get Go64...

https://www.stclairsoft.com/Go64/

If Apple follows past common practice, High Sierra will be off of support with the release of Big Sur arriving this Fall (Northern Hemisphere). Mojave will be the oldest release getting fixes.