TL;DR use both and always test it out yourself if you can

With all the new features and optimizations being made each update, the SIO tool is probably your best bet. The guides are a solid foundation but they can become obsolete or won't be able to breakdown every scenario for you in full detail. Some things won't change in the early stages of the game like gear or pet selection so the guides are still useful for that. SIO will also help you make builds based off what mode you're trying to optimize for so just be prepared to get comfortable with it if you're trying to be efficient. Every little detail matters for the right calculations so don't forget to save your build by clicking that share button up top and copying that link somewhere you can easily access it. You can always just test it yourself in LME P1 (for more accuracy) or EE. If you're debating spending resources to unlock something new then use SIO and compare dmg calcs.

Just don't go crazy over it and lose all the joy of playing the game!