From what a salesman told me, the cause of sail failure is the moss that grows in between the fibres, and wear and abrasion against rigging
. The 'plastic' sails hold their shape almost indefinitely but are harder to patch and repair. Try and get advice on what to carry on board if you are cruising.
The moss can be controlled by taking sails below every few weeks, in the dark to kill off the moss. The shape will then be good for much longer.
Another factor is that they are stiffer and don't like being crinkled and crushed so consider your existing sails, do they drop nicely, fold (flake) down nice and easy.
sail too, does it rub on every tack.
This is also the opportunity to consider a battened main to increase area. This can upset the balance of your boat (sideways through the water) so an increase in Fore Sail area is also worth consideration. Also think about reefing options, three reefs
have their uses with fore sails to match.
Given the opportunity that you have at the moment is it possible to get a matched set of fair weather
sails (prefered) and a second set of medium to heavy weather
sails (less area, heavier material, different cut) for when you feel the need to move on inspite of the prevailing/seasonal weather.
From MarkJ's comments a good assymetric? spinnaker
might well get a lot of use but might be not much better than a fair weather fore sail (more area and a different cut) that will have a wider useable window.
From your sail maker's remarks I'd go with the Warp-Drive. Your boat will look better, sail better, and the shape should last longer. Your old sails at 20yrs should have been re-cut at least once. If you have a lot of motoring to do the extra costs in sails will pay off long term, even on the inevitable day when you trade
her in. The daily run figures will still be better with the 'posh' sails.