Hey, Steffy,
The question about which the airline will let you on is actually a question on the airline, as some allow this even with bulkier models, and some don't allow this at all. If you're asking about which is smaller, it's probably the Dune, however they are very similar after folding, so that makes not that much difference.
I personally cannot recommend either - he reviews about the maneuverability and sturdiness aren't the best, while the strollers aren't that small after folding even with such small wheels. If you are searching for a full-size but compact model, the Cybex Mios, for example, is actually a much smaller but also better model - but that is just one option.
I hope I answered your question, even if I couldn't give you guarantees - the fact of taking the stroller up until airplane is up to the airline and you need to check with them as each airline has this set up differently, and the Dune is the very slightly smaller one here. None of the two SC models you're thinking about I would, however, recommend - make sure to check reviews for these before buying.