The C&O is definitely not awful, which is why I spend so much biking and hiking time on it, but it does have 2 major negatives and one minor one:
- Many sections take a long time to dry out after rain. If you don't like riding in mud, not going to be fun after or during heavy rains.
- It is run by the National Park Service, whose budget seems to constantly be in flux and rarely focuses on maintenance. A volunteer group I'm in (C&O Canal Association) provides funding and volunteer support but washouts and other issues on the C&O Canal Towpath often take years to get fixed or even temporarily bypassed.
- Minor one to me, bigger one to my wife: like the Appalachian Trail, the C&O Canal Towpath is mostly a long green tunnel. I like that, my wife said it got boring after the first 30 miles south from Cumberland.
The GAP doesn't have any of those problems, or at least much lower level of those problems:
- The surface drains better and was engineered better for biking - but there are areas that flood and stay wet for a long time.
- Problems on the GAP seem to get fixed much faster. This summer a small tornado moved through the Ohiopyle/Confluence section and dozens of trees came down - they were cleared within a day or so. Ditto a huge boulder that came down east of there - a workaround was in place almost immediately.
- Some sections of green tunnel, but much more variation.
I live closer to the C&O but do the GAP once a year. If I dropped into Cumberland MD with my bike and a credit card during a rainy summer like this one, and could only do one, I'd do the GAP. In a normal or dry year, it would be more of a toss-up - luckily, each year I get to do both...