Is the Ultimate upgrade worth the extra money?

It depends on how you play. It's worth it if you want to start with a full garage, custom weapons and four businesses open, and plan to spend a lot of hours in Leonida after the story. It's not if you only want the campaign: nothing suggests Ultimate changes the story, and you don't need it to finish the game. If in doubt, buy Standard: the upgrade can be bought whenever you like.

Checked by the GTA 6 Guide against Rockstar, Take-Two, official stores and the press. Sources for each fact are on the credits page.