I had already spoken to North Sails on the south coast, but Chris also posed the question to his local rep Jim, in Scotland. This is what he replied:
"Hi Chris,
The G-Zero will be the easier sail to use but the G-1 will enable them to sail lower angles downwind, the G-Zero generally works well down to 140 true wind angle but the G-1 can go down to 160 TWA.
The G-Zero would furl bottom up with the cable in the luff of the sail and operate in a similar way to a large genoa. The G-1 would furl top down with the cable outside the sail, this system does require more pulling on the furling line and can be more prone to user error.
The recommended sail cloth for both sails would be Norlon 150 (1.5oz nylon). My feeling is they would be best off with the G-Zero if they want ease of handling.
Kind regards
Jim"
I have also posted a question on the Cruising Association website and got some useful feedback. In summary, the G0 would be easier to handle day to day, and for that reason we have decided to order one with the bottom up furler.