I am not sure that I would consider this to be a mixed conditional sentence. Mixed conditionals are of two types. The first shows how an alternative past could have affected something in the present. The second shows how a hypothetical present might have had an affect in the past. This sentence is focused on the future, so neither of these should apply.
In my opinion, this First Conditional sentence. First Condition applies at any time in the present or future where the condition is real (i.e. quite possible but not confirmed). First Conditional sentences typically have the conditional clause in the simple present tense, and the main clause in the simple future tense. Assuming I am correct, both versions of the sentence would be considered to be wrong. The correct version would be:
I am free next weekend, so if you choose to go out for the evening, I WILL BE more than happy to help you out. (You could probably omit 'so' if you use a semi-colon after 'weekend'.)
As is often the case with questions of this type, the answer that I have given depends on the application of formal grammar rules. It is worthwhile using these if you are writing an academic paper, or wish to make an impression of being well-educated. Native English speakers, when speaking informally, frequently ignore some of these formal rules, and could possibly say either of the two sentences provided.