Derivative claims and s994 claims are similar. You can get similar remedies. In fact, one of the possible remedies for a s994 claim is a derivative action (this is because the court doesn't want you to have to re-start the litigation process all over again if it turns out you should have started a derivative action instead of a s994 claim).
The difference is that derivative claims are claims by the company. The shareholder represents the company in the litigation on behalf of all shareholders, and any remedies are awarded to the company. Section 994 claims are claims by that particular shareholder and any remedy is awarded to that particular shareholder. In any given scenario, think about what the claimant is trying to achieve - is the claim about protecting the company as a whole or is it about protecting that particular shareholder, or class of shareholders.