I'd be wary of wil/going to because it could be opening a can of worms in an observed lesson as it's not that simple- I think I'm going to be sick, not will- which raises the issue of evidence now & decision now, in addition to plans, etc. There's also a grey area where both are OK. If you want to do a grammar point, then I'd choose something that is more binary, and therefore less likely to raise problems in the short time you have. However, do yo have any clues about their ethos and what they are expecting? If they are full-on communicative approach people, then grammar might not be the best approach. What do they say in their literature?
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