Given this is in the CX section and the other post mentions that he is spec'ing his CX I think we are talking about the modern mogs? I cannot speak for the CX suspension but for the Aero ?

I have an Aero S2 on 18's and a Coupe (slightly heavier) on 19's with the same outside diameter and a very small sidewall.

On the 18's I have more sidewall and I think the overall balance of comfort, grip and cornering is better. I am also less worried about the rim being damaged because of no sidewall and the range of potholes we have around here.
I would also say the 19's are more pressure sensitive and you can feel it through the steering and in the handling quickly.

Having said that the 19's are the Rays aeromax design and I think they match the Coupe so they are not being changed.

I can get most of the decent tyres in both sizes now so no restriction on that front.

Everyone loves a Morgan. Even me, unless it's broken again.