1. I'll bring it to you tonight.

2. I'll take it to you tonight.
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Which one is grammatical? Are both OK?

Both are correct, and mean approximately the same thing.

Usually 'bring' means toward you and 'take' means away from you, but in these sentences, the difference is hardly noticeable.

Both sentences are grammatically correct, but they have slightly different meanings.

1. "I'll bring it to you tonight."
In this sentence, the speaker is indicating that they will personally bring the item to the person they are addressing later in the evening.

2. "I'll take it to you tonight."
Here, the speaker is implying that they will transport the item to the person they are speaking to. The verb "take" suggests that the item is currently with the speaker and they will deliver it to the other person later that night.

Both phrases are acceptable, but the choice between "bring" and "take" depends on the context and the location of the item in relation to the speaker and the person they are addressing.