Which cutting style avionic snips do I choose if I have to choose just one?
Right snips are the most often used and it is possible to use only them, but if you want a neat and professional finish I would advise getting both right- and left-cutting, considering they are not that expensive for a decent set. 

If you are a beginner with corrugated it may be worth getting offset snips if you're planning on cutting the length of sheets. If you are only doing penetrations or trimming corners etc. non-offset ones are the easiest to learn.