The original question is "which has the bigger downside". I am extremely reluctant to give advise about what length is best. Its different for different skis, styles and objectives in skiing. I ski on boards ranging from 168 to 193 and waist sizes ranging from 68 to 94 mm; and they all work fine for what I want to do, and I feel I know how to match the task to the ski.
People come on these boards and ask "what length for me?" Once we know what ski they want (performance parameters), describe their skiing style and terrain, and objectives, they usually get a range of advice and wonder why there is no concensus here. If you need to ask what length is appropriate for you, I will probably err on the short side. The question is often accompanied by qualifiers like "I am a strong skier but prefer short turns" "Need to go down tight trees and chutes" All of these statements are begging for a short ski answer, or are another way of asking "can I handle a longer ski?". A confident, fast, charging skier does not ask the question.
Now the original questions: Putting a skier on a shorter ski IMO poses less risk than putting them on a longer ski. The shorter ski is easier to manuver, will generally gain less speed and has few drawbacks until it reaches its upper speed limit. This speed limit is probably outside the comfort zone of the person asking the question, and is not much lower than the ski of the next available size. So if someone asks, what size ski do you like...I can answer, "depends". If they ask me what size they should like, I will probably go short for their own benefit with a recommendation to demo. Most of us can ski multiple lengths well. A response should not infer a shorter ski is appropriate only for a weaker skier; you will just make the Metron owners mad.