As for what kind of "language" to use:
In all honesty, I'd stick with what's comfortable. And by that, I mean make yourself sound like a human being as opposed to an essay-regurgitating machine. There's isn't enough time or "weight" in EE2 for you to use as many multi-syllable words as possible - they really need to hear the basic gist.
With research, my medium is short story buti changed to that from critical, so i am discussing how i researched for both and how my skills of research i developed from critical have helped in the different medium.
They are EXTREMELY likely to ask *why* you changed medium. I hope you're preparing for that question too!
I'm sure I've said this elsewhere, but back in my day my teachers gave us ONE yellow palm card and said that was the only thing we were allowed to bring into the room when we did our Vivas - emphasis being on speaking directly rather than coming in with a prepared speech. For all of it I spoke very casually, and found the whole process quite enjoyable (in fact I was greatly disappointed when time was up, I wanted to keep talking on and on and on about my work
). Most people say the same thing.
As for questions: Don't freak out about it
This is not eng adv of EE1 where you're supposed to memorise a bunch of stuff and then regurgitate it. They're only going to ask you about your major work - stuff you can't possibly help but know inside out as you wrote it and researched for it entirely by yourself