Trail surface will make a major difference in your choice of bike. You might be interested in our trip report . On the trails of the Allegheny Trail Alliance, between Pittsburgh PA and Cumberland MD, the trails are built on railroad grades. The finished sections are mostly smooth packed crushed limestone.
Some sections are better, some are a bit rougher.
The few gaps in the trail are paved, but they have big hills. The towpath east from Cumberland MD gets sloppy in the rain with puddles in the tracks and a few muddy sections, but it's still passable. We rode from Paw Paw WV to Hancock MD in an all-day rain on Memorial Day weekend a couple of years ago. We were soaked, but we didn't have trouble riding. In case of really heavy, torrential rain, certain sections of the towpath could flood, though.