I see your predicament,,,a real tough call. I think I would leanmore towards filling the width of the tubs as apposed to having more sidewall, more rubber to the road :)
Rod,
Since I figured it was time to put the Blizzaks on my Charger for the winter, I decided to see what one of the 20" wheel/tire combinations would look like on the Duster, since the tire is 28.5" tall it would be close to the height of what the Mickey Thompson S/R would fit like on a 20" wheel...obviously the new wheel would have a totally different offset but it was enough to visualize the "look". I can't say I was Sold due to the lack of sidewall height, so it looks like I'll stick with the 18's.
On a side note I re-measured the current M & H on the 17" x 11" wheel just to see how close it was to there advertised specs since they measured it on a 12" wheel, section width was 13.4"and the 18" closest tire is actually 13.8" of course that's measured on a 13" rim.....so It will be real close in width just a little shorter.