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What would you say? I´ll speak to you tonight or I am going to speak to you tonight, or speak to you tonight.
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"I will" is perhaps a little less formal, and certainly more brief.
I will speak to you tonight; I intend to speak to you tonight.
I am going to speak to you tonight; I am proceeding toward a point in time (tonight) at which I intend to speak to you.
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I'd most likely say the first.
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why?
 Mike in Japan's reply was promoted to an answer.
The second one sounds terrible to me. What about negatives?

-I am afraid I won´t be able to talk to you on Thursday.
-I am afraid I am not going to be able to talk to you on Thursday.
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Again, I'd say the first.
The second isn't wrong or anything. It's just that people tend to say the first more often than the second. Beats me why.