Raschmad, Fit is a secondary consideration in the choice between PCM and RSM, particularly if items with similar threshold patterns are grouped together using ISGROUPS=. This is because differences in fit tend to be dominated by accidents in the data. Please see http://www.rasch.org/rmt/rmt143k.htm
Suggestion: model the data with PCM, output the person measures (PFILE=)
model the data with RSM
cross-plot the two sets of person measures
What difference has the choice made to the person measures?
If the difference makes no substantive difference, then RSM
1. If you decide on PCM, then communicate to your audience the substantive implications of the differences in the thresholds.
2.Are you sure that "threshold ordering = ability to distinguish between categories"?
Surely: "fit of responses within category = ability to distinguish between categories"
"threshold ordering -> width of category on the latent variable"
Where strong disordering = narrow category, and strong ordering = wide category.
(Remember that thresholds are pairwise between adjacent categories, not global across all categories. If you want to think about global thresholds then please use Rasch-Thurstone thresholds, instead of Rasch-Andrich thresholds.)