Start with one general assistant and use it daily until you know its strengths. Move to a paid plan only when you regularly hit limits or need features that matter to your work. Check each vendor's current plans, since they change often.
Add a second tool only to fill a clear gap, such as a different model for a second opinion, or a coding assistant if you write software. Pay-as-you-go APIs make sense once you automate repeated tasks. Local models are worth a look when data must stay in-house. Before adding anything, track for a month which tools you actually open. Most people need fewer tools than they think.