I don't have answers for you, but I can give you some questions to ask. Once you get the answers from your salesperson, you can decide for yourself.
Both cloths will make good sails, but differ in their resistance to flogging, mildew and UV. They will also differ in their capacity to resist stretch during use and over the service life of the sail.
Resisting stretch when a gust hits is important always. Resisting permanent deformation due to stretch over the lifetime of the sail is also important, and is a separate but related issue.
What is the difference in price between the two, and what's your budget like?
Is the higher cost of lower stretch and less easily deformed sail may worth more to you in terms of performance?
Both the 3di Polyester and the Radian polyester will retain a good shape for longer than a sail made of dacron and with cross cut seams.
Ask your sales consultant "Which one will have a better shape in 3, 5 and 10 years?" Compare that to how long you plan to own the boat.
Ask which one will last longer in UV, if you plan to take the boat to a high UV destination,
Ask which one resists mildrew better/worse, if you live in humid climate, or where it where it rains frequently.
I don't have any experience with the relatively new NorDac 3Di sailcloth. I think it's been available since July 2017 so long term reviews aren't available. I don't know how durable it would be over the long haul with regard to humidity or UV or flogging. Since the 3DI relys on resin to hold the fibers, I suspect the service lifetime will be shorter than for woven dacrons (for radial and cross cut designs).
If mildew is a concern, I think the radial-cut dacron, Radian, will dry faster than the 3DI NorDac, but check with your salesperson.
Have you compared the cost to Norths' panelled cruising laminate, NorLam? I would suspect, but I'm not sure, that a NorLam tri-radial sail (cruising laminate with dacron skins over mylar skins ) will hold its shape as long as the 3DI NorDac dacron, and at a significantly lower cost. The shape will be a little bit stretchier, but still very good for performance cruising. And the cost will, I bet, be significantly lower.
Judy B
I have a quote from North Sails replacing the original set and they are suggesting 1 of 2 sail materials.
3Di Nordac 330 or NPC Radian .
Anyone have thoughts on the material or North sails as the replacement.
Both are furling.
Thanks Bob