Since your crew would be doing the mainsheet upwind, why not go for a 8:1. This gives the crew more reaction possibilities in a big gust and since he/she has 2 hands to sheet in 8:1 is sufficient in my opnion.

On the other hand the skipper has 8:1 -> more finetuning downwind, because 3:1 seems a little difficult to trim the main to me.