[rhagfo]

Interesting your new 5er has a GVWR of 12,300#, you are not likely
Grossly over GVWR.
So now to the next TV, you will not be in a payload pinch so tough choice. The F450 comparably equipped will have less sticker payload than the same F350.
The reason both the F350 and F450 have a 14,000# GVWR, Ford does this to the F450 pickup to make insurance and registration less costly.
The F450 is a stronger and heavier truck, that is why the payload is less. If you plan on going bigger in the future get the F450.