Welcome to the forum.
The Sigma engine in the 4/4 is a a free -revving delight , but to get the best of it you do need to rev it heartily. If you are not used to that treatment, then an engine that produces more torque may suit you better, but it will be different, not so free revving, and that is important in a sports car , and especially a Morgan. The 4/4 is also more nimble, and easier to handle , so don't get concern yourself overmuch with the engine power/torque figures.
The Duratec and GDI engined +4s are both great to drive and will give more torque than the Sigma BUT neither can match the torque figures of a turbo-diesel so try both.
If you really want lazy torque and very long legs then a Roadster or an older +8 may suit. It is important that you try each one over a varied route , and make your own mind up . These are physically bigger and heavier cars so the extra torque does come at a price, the handling is less nimble and you have more weight transfer in corners.
As others have said before , the right car will choose you.
1999 +8 , Indigo Blue.
Ex-John McKecknie/Mike Duncan 1955 +4 racer.